The fee-fi-fo-fum call of supermarket giants such as Fairway and Whole Foods heading to Brooklyn next year has done a lot to shake the nerves of the Park Slope Food Coop.

But the 32-year-old co-operative market may find its biggest competitor much closer in size: a corner gourmet grocery half a block away that offers organic produce without the monthly work commitment.

Union Market, the latest gourmet food store to swing its doors open on the brownstone-lined streets of Park Slope, carries a selection of organic vegetables and fruits, meats that are free of hormones and antibiotics, products from regional farms, and more than 150 kinds of gourmet cheese.

Although its prices are higher than those at the Food Coop – the co-operative’s Web site boasts up to 40% savings on groceries – the new market, which opened less than six weeks ago on the corner of Sixth Avenue and Union Street, serves prepared foods, delivers daily, and does not require membership – an advantage that may lure some Coop members to its aisles.

Members of the Food Coop pay a onetime fee of $25 and a $100 “investment fee” that they get back if they quit. Each member must also commit 2 hours and 45 minutes of volunteer work a month to earn membership discounts.

According to the co-op’s general manager, Joseph Holtz, the volunteer labor force allows them to sell goods at 30% less than supermarket rates.

“The temptation to switch is certainly there,” a co-op member, John Arceci, said. “Three hours every month or so doesn’t seem like much, but it can quickly become inconvenient.”

Mr. Arceci, though, said he doesn’t plan to switch to another store, whether Fairway or Union Market.

“It is really a compelling way to deal with what is otherwise a fairly mundane part of life,” he said of the co-op. “It becomes about more than shopping.”

Another co-op member, who declined to give her name, was less enthusiastic about the work. “It is like a fascist bureaucracy sometimes,” she said. Ever since Union Market opened in her neighborhood, she has been considering leaving the co-operative, she said.

Mr. Holtz, though, is not worried. He said the reason cited by most members who leave is moving out of the neighborhood. Others cannot fulfill the volunteer requirement. He said membership has doubled in the past five years, reaching 12,000, and sales are up 14% from 2004.

A co-owner of Union Market, Marko Lalic, said he does not see a rivalry brewing between the local markets.

“Like the Coop, we are a business, but we cannot compete with their price structure,” he said.