Once a year in Jacqueline Kniepple's native town of Lille in northern France, "the entire city turns into a flea market for 24 hours," said the Rockville resident. "Traffic is blocked off and people set up their stands in the middle of the road, peddle their wares and feast on clams and french fries."

The closest thing Maryland offers to this Brigadoon of flea markets is the Rockville Swap Meets.

"They are one of my greatest joys," Kniepple continued. "You can find everything there under the sun."

John and Judy Mathwin, also of Rockville, agree. They calculate that during the 14 years they have been attending the swap meets, they have saved thousands of dollars by purchasing recycled clothing for their sons, now 15 and 17, as well as toys and household items. At a recent swap meet, John Mathwin, an English teacher at Montgomery Blair High School, searched for books of essays to use in his classroom.

Sponsored by the Montgomery County Department of Environmental Protection, the Rockville Swap Meet -- fondly dubbed the "granddaddy" of local flea markets by aficionados because it is one of the longest-running in the county -- has fulfilled its mission of "turning trash to treasure" for 18 years.

Every fourth Saturday in April through October, from 7:30 a.m. to 1 p.m., the four-tiered County Council Office Building Garage at Jefferson and Monroe streets is transformed into a vast marketplace by 250 to 300 sellers and as many as 2,000 buyers. The last swap meet of the season will take place Saturday.

On swap meet mornings, parking spaces customarily reserved for the county executive and chief administrative officer are usurped by vendors hawking items ranging from Waterford crystal vases -- new from the box -- to hand-carved mandolins to recycled gardening tools. Negotiations that rival congressional budget hearings in their complexity erupt over the value of merchandise displayed on blanket-covered car hoods, folding tables and clothing racks.

"Rockville is a prime sale because it attracts so many serious shoppers," said Jim Flanagan, a vendor. On the mornings of the swap meet, he and other vendors start lining up their cars around the garage at 4 a.m. to get a good location for setting up their wares.

"You can always identify the really serious shoppers because they are the ones who line their shopping carts with plastic clipped into place with clothespins," a tactic to help protect anything a shopper might throw into the basket, Flanagan explained. "These diehards will shop until the wheels fall off their carts or they run out of money, whichever comes first."

The monthly flea markets also serve a social function. "You meet your neighbors there and walk around and visit while you shop," said Mary Schaheen, of Derwood, at a recent meet.

The Rockville Swap Meet was introduced in 1972 by Jacqueline H. Rodgers, then a planner with the Department of Environmental Protection. Rodgers, secretary of the state Department of Housing and Community Development since 1987, recently recalled, "I had recently relocated from Southern California, where my parents enjoyed the swap meets, which were in vogue on the West Coast. Montgomery County was interested even then in environmental issues and the swap meets were successful from the beginning."

Rodgers, who supervised the swap meets during their first season, "quickly tired of junk" and hasn't been back to one of the sales since. But she said she still treasures a used Sears bicycle that she purchased at the swap meet for $25 and keeps at the beach.

In 1982, when budget cuts eliminated county staffing for the swap meets, their operation was contracted to the Rockville branch of the Civitan Club, an international service organization that raises money for the mentally retarded and physically disabled.

"The local members do a wonderful job of running the sales," said county liaison Marion Smith, who has amassed an impressive collection of Coca-Cola memorabilia during her 14-year affiliation with the swap meets.

Fritz Moreland, president of the all-male Civitan Club chapter, said all proceeds from the 25-cent admission fee and $4 base charge to vendors go to local charities. Each swap meet generates a profit of $1,500 to $2,000, Moreland said. This year, with the sales as its primary fund-raiser, the club donated $22,000 to 12 organizations, including Gaithersburg HELP, which assists residents with emergency needs, and Mary's House for the elderly.

Montgomery County also sponsors swap meets from 7 a.m. to 3 p.m. every third Saturday of the season in the public parking lot at Pershing Drive and Fenton Street, in Silver Spring. These sales, sponsored for fourteen years by the Silver Spring Center and Department of Recreation, Eastern Area Office, attract a good crowd, according to on-site supervisor Tina Clarke.

Clarke adores the sales not only because they have been the prime source for her acquisition of pink Depression glass, but also because she enjoys the community spirit they foster.

"You get the same dealers back year after year, and you come to think of them as family," she said. "The last swap meet in October is always a sad time. There is lots of kissing and hugging and saying, 'Goodbye, good luck. Have a merry Christmas and a happy new year. Stay well, and if God wills and the creek don't rise, we'll see you all next year!' "