When Kent and Mary Stevens send out invitations to their fall party, they tell their guests to dress warmly and bring knives.

Oyster knives, that is.

Those who did were prepared for the chill -- below 30 degrees -- and the attack on 14 bushels of Chesapeake Bay oysters offered raw or cooked last weekend at the Stevens' third annual oyster roast in Leesburg.

Few guests seemed to find it odd that 100 or so people in hats, gloves and jackets were gathered in a backyard on a cold November night while the lamplight glowed warmly from the windows of the Stevens' two-story, empty house. They were too busy eating oysters, even those who had neglected to come armed.

"We forgot to bring knives," said Sally Tilson, who lives near Lucketts. "Every now and then I find a knife and madly open them," she said, standing near a long table laden with heaps of steaming oysters.

The guests were mostly from Loudoun County, the oysters from the Chesapeake Bay. The tradition of outdoor oyster roasts came from Virginia's Tidewater, along with the Stevens family, who moved to Leesburg about four years ago when Kent became administrator of Loudoun Memorial Hospital.

"She's the one who got me interested," said Kent Stevens, gesturing toward his wife, a native of Suffolk, Va. "Twelve years ago you couldn't get me near an oyster."

But when they lived in Franklin, Va., where Kent was an administrator at the hospital, the family started attending a friend's oyster roasts. Kent remembers his first raw oyster: "I had to just take it and dunk it, just absolutely douse it in cocktail sauce."

Mary Stevens admitted that she, too, is a late bloomer. Although her family regularly enjoyed oysters, she didn't develop a taste for the mollusk until adulthood.

When they move to Leesburg, they decided that a tradition good enough for Tidewater was good enough for Loudoun County. Kent figures proof of this is that, in the third year of oyster roasts here, "the consumption per person has gone up." (He allows about one bushel, or 200 to 250 oysters, for 10 people.)

"I think," said Mary, "we've convinced a lot of Loudouners to try their first oyster."

Certainly Loudoun residents have been eating oysters for a good many years, and oyster stew, baked and stuffed oysters, as well as straight from the shell have been traditional Thanksgiving fare.

Many guests, though, including Kent's friends from the hospital, real estate friends of Mary, a Realtor, as well as neighbors and other friends from throughout the county, said the wintry outdoor oyster roasts were a new experience.

"Most assuredly," said Brent Lagergren, director of planning and marketing at Loudoun Memorial, who moved six weeks ago from Kansas City, Mo. In Kansas City, said Lagergren, oysters "come open" on a plate carried by a waiter.

He'd bought himself an oyster knife, however, and began to tackle a tightly shut raw oyster. "This is off the record," he said, swiftly carrying his oyster to a far table and turning his back on his audience.

Mid evening, as the temperature dropped below 30, guests moved back and forth from the tables under the tents to small fires around the edges of the lawn, or gathered near the oyster cooker, a cinderblock and corrugated metal affair on which the Stevens' nephew, Karl Bock, heaped a half bushel at a time, then covered them with wet burlap. From the heat of burning logs underneath, the oysters sizzled, steamed and emitted an intoxicatingly delicious seafood odor.

Vying with the odor of roasting oysters was the scent of vegetable soup. Mary Stevens had made 30 quarts, for guests not dedicated to the proposition of eating oysters.

This was, in part, to disprove the notion that some guests might have that the affair was strictly "a serious gathering of bivalve lovers," said Kent.

"Oh, I didn't know that was why people were invited. Were we invited to eat oysters?" laughed Peggy Wenner., who own an orchard in Round Hill. "We just got back from Boston at six o'clock tonight," Mary said. The Sleeters were attending a convention, and had planned to stay another several days. "But we wouldn't have missed this for the world," Mary said.