When a tractor-trailer truck filled to the brim with watermelons pulled into parking lot 6 at RFK Stadium one day last summer, a small group of local farmers who paid $120 a month to sell their produce at the D.C. Open-Air Farmer's Market eyed its North Carolina tags with suspicion, doubting that the watermelons came from within the required 250-mile radius of Washington.

Farmers began asking questions. The result is a reorganization, a new set of rules and a better accounting system at the market, which opens its fourth year today.

Before last year, farmers had little involvement in running the market, which is administered by a council including representatives of the United Planning Organization, the Mayor's Commission on Food, Nutrition and Health and the D.C. Cooperative Extension Service, along with a number of advisers.

"Farmers are more concerned with the weather than they are with politics," said Jarvis Cain, an adviser to the council.

But the farmers saw out-of-state produce as a threat. They demanded an accounting of the $24,000 farmers had put into the market operation in association fees and rent, as well as of a $50,000 grant from the city. They said they found an inefficient accounting system, fluctuating rules and a fee schedule that was not being uniformly applied.

It was also discovered that some farmers were buying more than the allowed 20 percent of their produce from wholesalers or other farmers, rather than growing it themselves.

Market coordinator Alfred Smith reworked the system over the winter, and now, according to Cain, farmers are looking to the market's future with "guarded optimism."

The market's first audited financial statements were released to farmers in February. Committees have been set up to arrange publicity for the market, an effort which had lapsed, and to develop well-defined rules and regulations and to plan for the market's expansion, Smith says. As for the hucksters (vendors who don't raise the produce they sell), "we have a commitment to get fresh fruits and vegetables to people in this city. That's our first priority," Smith said. Farmers have a right to bring fresh fruits and vegetables from wherever they can get them, he said, adding that "what we don't want is farmers using the market as an opportunity to become vendors."

Farmers had complained that no-show fees for farmers who did not show up were not being evenly applied. "This year everyone pays the same whether they show or not," Smith said. "The time for being flexible and making adjustments is over. You have to be consistent."

Ranked among the top 12 of 1,500 similar organizations across the country, the market last year grossed close to $1.2 million for the estimated 14 tons of produce that changed hands, according to another council adviser.

Smith sees the RFK market becoming a large umbrella operation, with satellite markets throughout the city. Officials are looking at sites in Georgetown and Anacostia, he says.

"We're still toying with where this program is going," Smith said. "It will become a community enterprise," he said, with responsibility for its management in the hands of community groups. "But right now . . . this program is still at a tentative stage."