IT'S two days before Thanksgiving, and the Park Slope Food Co-op is straining at the seams. Organic turkeys reared in upstate New York are spilling out of the refrigerator, boxes of pumpkins and butternut squash grown in the Hudson Valley block the aisles, and check-out queues stretch round corners and double back on themselves. It's not a good place to get your shopping done in a hurry.

It could be any supermarket struggling to meet demand on one of the busiest days of the year, except that nobody seems to be rushing to get anything done. A conversation at the till begins with, "Is this the regular avocado or the organic?" and continues into an exchange of guacamole recipes; workers pause from stocking the cheese cabinet to discuss which of the six varieties of cheddar has the most bite. There is little urgency and still less boredom in the air.

Nobody on the shop floor is being paid to work here. None of the shelf stackers have been grimly accumulating the minimum wage since five in the morning. All are members of the co-op, honouring their commitment to a shared enterprise. Almost 13,000 people shop here, but there are only 57 members of staff, meaning that more than three-quarters of the work is done by members. It is, by some distance, New York's cheapest wholefoods store.

Joe Holtz is the co-op's general manager. In 1973, he and a group of friends living in the same Brooklyn neighbourhood, tired of shopping at supermarkets they viewed as overpriced and ethically unsound, decided to set up an alternative. "None of the founders would have described it as a commercial enterprise," he says, "because we were a group of pretty anti-business people, but we knew we were trying to start an institution. For the last 25 years I've been using the word business.

"It grew very rapidly in the beginning. Within a few weeks we had a few hundred people coming here every Saturday to buy food, and we tried to sign those people up. We had a work chart with all the jobs listed for the following week - helping to receive deliveries, helping to run check-out lines or to stock shelves, helping to clean. We tried to explain that this was a co-operative and we hoped and expected that you would sign up, but it didn't work. People were very happy to come and shop, but not to sign up on the chart."

The early Seventies was a boom time for the co-operative movement. The old wave food co-ops founded during the Great Depression of the Thirties had gradually been assimilated into the retail economy, and a new generation of activists schooled in the civil rights struggle and opposition to the Vietnam war were seeking to re-energise resistance to the capitalist hegemony. As a member of Washington DC's Field Of Plenty Co-op told a reporter, "Selling food isn't our goal. It's just a pretext for building living and breathing models of revolutionary change." The majority failed within two years.

Those that survived gradually went the way of their predecessors, and indeed Britain's own Co-op food chain, becoming collectively owned grocery stores that anyone can shop at. There are still more than 300 food co-ops in the United States, but Park Slope's is the only one that insists on participation as well as investment. It costs $25 to join, plus a $100 stake that is refunded when you leave. Each member must work one three-hour shift every four weeks. Miss a shift without calling in first and you have to work two to make up for it.

"All those co-ops believed that co-operation meant working together, and yet our co-op stuck with it," says Holtz. "The other co-ops didn't stick with it because it didn't work. And it didn't work here either, but we were willing to make rules that said You must come, or you can't shop' and I don't think other co-ops made those rules. The flower children and hippies were definitely not into making rules.

"Our members do 75% of the work. The biggest expense of running a store is paying for labour and so we've eliminated a lot of paid labour. Also, if you expect people to care about the co-op, and truly feel like they own it, then it means working together, it doesn't just mean investing together.

"There's two equities here: there's sweat equity and there's money equity. The sweat equity is probably the most important ingredient in our co-op, because it makes people feel a sense of belonging and caring."

WHEN I moved to Park Slope recently, the first thing my friends from Brooklyn asked was "So, are you going to join the co-op?" Many were sceptical. Several threads on internet message boards detail the institution's perceived failings, including one particularly heated debate at foodie site, chow.com, which starts with a description of the co-op as "Something between an earthy-crunchy health food haven and a Soviet-style re-education camp" and goes on to accuse its members of being "lazy and snobbish", "Stalinesque" and "authoritarian-elitist-arrogant". I signed up right away.

There are two main strands of criticism. The first alleges that the co-op is run by die-hard vegans who refuse to handle meat at the till and tut disapprovingly if you use plastic bags. "There probably are some members out there who are smug and self-satisfied and look down on non-members," Holtz admits. "We have members who are obnoxious and we have members who are wonderful."

But the most common objection concerns the work requirement itself, which is frequently denounced as unnecessary, unfair and inflexible in its implementation. People complain that squad leaders' and the internal hearing and deciding' committee refuse to make allowances for illness, domestic crises or family bereavements. Enough disenchanted ex-members have aired their grievances for the co-op to be voted the "best place to experience how communism turns to fascism" in the Village Voice in 2004.

"It's a bad idea to join the co-op if you're not committed to the work," Holtz says, with a shrug, "because you get behind on your work and eventually you're told you can't shop, and it feels bad. I think there are progressive people in our area who are not comfortable with not being members because they feel a statement is being made about them."

The work is hardly back-breaking. Assignments include data entry and answering phones in the membership office, unloading delivery lorries, pushing a shopping cart for members who live within a six-block radius, stacking shelves, guarding the exit or running the on-site crche. The rule book says every adult in a household must join, but allows people to work for their partners, inevitably creating squads of co-op widows' doing time for their husbands.

For people who work long days in an office with few opportunities for flexitime, it can be tough finding a convenient slot in the rota, but as a freelance journalist living five minutes' walk from the co-op, I could essentially choose whatever shift I wanted. I opted to spend every fourth Tuesday afternoon down in the basement on food processing' duty.

At first, this involved taking dried figs from a box, transferring them to clear plastic bags, weighing them and taking them upstairs to the shop, listening to old ska compilations and the best of the Everly Brothers on the stereo, and making polite conversation with the largely middle-class women in my squad.

On my second shift, I was promoted to cheese, due to a shortage of qualified personnel. A quick scan of the counter upstairs revealed stocks of gruyere, gorgonzola and manchego to be dangerously low, a potential flashpoint during the holiday season. We set to work, in leisurely fashion. "There's no cracking the whip," squad leader Cherie Stein reassured me, "I don't believe in cracking whips."

The pay-off for three hours of extremely relaxed labour is the opportunity to shop at just 21% more than wholesale prices. As the co-op purchases a lot of its cheese direct from an importer who happens to be a member, this is much, much, cheaper than the speciality delicatessen down the street. Extra mature cheddar costs $5.82 per pound instead of $12. Parmesan costs $9 instead of $14.

Twenty-one per cent is the standard co-op mark-up, calculated to cover all outgoings and leave the bottom line as close to zero as possible. It means that some products are incredibly cheap, like the tiny bags of spices, which typically sell for less than a quarter of the price they would fetch packaged in plastic jars in a supermarket. Other items with no potential for saving on labour end up costing roughly the same. I priced a hypothetical shopping bag of tomatoes, mushrooms, avocado, aubergine, fresh herbs, bread, milk, orange juice, cheese and chicken at $35.19. At Wholefoods, New York's most established upscale grocer, the same basket would cost $47.09. The Wholefoods chain was founded in 1980 and now has 33 stores across the Eastern seaboard. A new branch within walking distance of Park Slope will open next year, offering consumers the chance to pay extra for their ecologically sound vegetables instead of working for them. As a test of genuine commitment to social enterprise, it could hardly be better.

THE co-op enables low-income families to buy healthy, high-quality food that would otherwise be out of their reach. Joining fees are substantially reduced for people on benefits. Park Slope is home to novelists Paul Auster, Jonathan Franzen and Jonathan Safran Foer - hardly a low-income neighbourhood - but the co-op's membership reflects a wider Brooklyn demographic and is by no means universally well-off.

Food that is still edible but no longer in good enough condition to sell is donated to local soup kitchens. Members earn work credits sweeping the streets four times a year, but the main plank in the co-op's socially responsible manifesto is its relationship with farmers. It buys local wherever possible, encourages low impact agriculture and organically certified farms, and always pays at or above market rate.

"We charge a tiny bit more for local produce," says chief buyer Alan Zimmerman, "and in return, our members get a farmer who's planting for us and is responsive to our needs. We're also getting to preserve the land around us by contributing to its welfare. For the farmer, we make a commitment to buy their food, and that's what a farmer really needs. It is an absolutely win-win situation."

AMY Hepworth sold her first watermelon at the age of five and was "an expert fork-lift driver" by eight years old. Her family has worked the land in the Hudson Valley town of Milton, two hours' drive north of New York, for almost 200 years. Her education at Cornell University equipped her to transform the farm into a leading organic producer when she took over in 1978, and she's since become a pioneer of integrated pest management, which uses the least amount of pesticides to grow bug-free fruit. She thinks "giving up insects was the worst thing that happened to mankind's diet" but knows from experience that consumers won't buy apples with worms in. A passionate maverick, she gives the impression that she rarely sleeps, and hardly needs to.

Hepworth Farm grows a huge variety of fruit and vegetables for the co-op, including squash, tomatoes, lettuce, beetroot, potatoes, pears, pumpkins and apricots. They've recently started growing okra, a high-risk crop because the price is right and demand is steady. "When they commit to you, you're secure," Hepworth says. "If Park Slope didn't commit to us, I probably would have given up. When you know you're gonna sell something and they're not going to drop the price on you, it makes you strong enough to do it.

"The only way to save farms is one at a time, and Park Slope know that, so now we're connected and we serve each other. They need arugula rocket in November, we grow it in November. We don't grow it in August because someone can do it better than us.

"Our invoices, we don't even put pricing on it, because they are so righteous and so concerned about the farmer that they sleep well at night. Their buyers don't have to think when they go home oh, I burned that guy'. They do the best for their customers, but they're completely protecting the farmer all the time. They don't try to get something for nothing.

"They always try to give you the fair price, so when the market goes up and they can't find any tomatoes they'll pay $30 a box. That's unheard of. We were happy with $20. But he says Oh, the market's strong this week so we'll give you $30' and that's why I never put prices on the invoices."

The contrast with the notoriously predatory buying policies of major supermarkets could hardly be more acute, but the co-op is not alone. "There's a movement in this country that is local and sustainable," says Hepworth, "Wholefoods wants to support local farmers now too. It's a popular thing, a marketing tool. People want to feel good when they're buying food. Wholefoods buyers do make commitments, and that is the future. You have to have commitments with growers because us growers are disappearing."

BACK at the co-op, the Tuesday shelf stacking squad are unpacking boxes of Amy Hepworth's broccoli. They're taking their time, as usual, and are constantly interrupted by people squeezing past with their trolleys. Turnover is seven times that of an average supermarket, so the vegetables are fresher, but the aisles are also permanently congested.

Cash and pre-paid cheques are the only ways to settle up, and shoppers queue twice: once to ring up the goods and once to pay. A recent meeting resolved to streamline the process by accepting debit cards but not credit, "because people felt that credit cards were politically incorrect".

At the regular orientations', potential members are asked why they want to join the co-op. For each person who says they're interested in social enterprise and serving their local community, at least three answer "because I want to buy cheap organic food". The organisation thrives as a business model first and a support network a distant second.

Nonetheless, some members are clearly taking home more than cut-price ice-cream. Debbie Parker started as a member six years ago and then took a job in the membership office. "What I found in this community was real togetherness and understanding," she says, "When we have things happen like the black-out a couple of years ago, our members were coming to the co-op to see what they could do to help. You didn't see it in other stores - they were probably knocking on the door to see what they could take."

Scotland's co-operatives fall into two camps: state-funded institutions aimed at improving inner city diets such as Edinburgh's Barri Grubb, or small-scale rural operations with a wider remit to preserve the quality of village life, like the Appin Community Co-operative in Argyllshire. None even approach the reach of the Park Slope Food Co-op.

Chief buyer Alan Zimmerman wonders why, after more than 30 years, their demonstrably successful approach still isn't catching on, at home or abroad.

"I see no reason why it couldn't work in other places," he says. "I don't think our model has been abandoned but it is scary for people to try it this way. The member labour requirement is daunting." On the shop floor, the woman stocking organic potatoes whistles while she works, undaunted.

For more information: www.foodcoop.com