The 2-year-old Prince George's County girl bitten by a copperhead snake near La Plata, Md., on Sunday remained in serious condition yesterday at Children's Hospital in Washington, but her condition is improving and she is expected to recover, a hospital spokesman said.

Lisa Maria Thompson, of 3337 Walters Lane in Forestville, was moved from the hospital's intensive care unit yesterday, hospital officials said.

The girl was bitten on the right leg while playing with her sister in the back yard of a family friend's home on Fairgrounds Road in the rural Spring Hill area of Charles County.

Copperhead bites are rarely fatal, snake experts said yesterday, and the girl's recovery was virtually guaranteed because of fast work by health professionals. She was rushed to Physicians Memorial Hospital in La Plata, treated with antivenin medicine and flown by Maryland state police in a helicopter to Children's Hospital, accompanied by her mother, Betty Thompson.

"We were all in the yard, and my daughter was playing with her sister," Betty Thompson said yesterday. "It bit her, and she screamed. I don't think she knew what it was."

The friend the Thompsons were visiting, Sam Swanns, killed the 26-inch snake with a large stick and took it to Physicians Memorial Hospital, where it was identified as a copperhead.

"The first thing I knew was to get her to a hospital," said Thompson, who works on a housekeeping crew at Prince George's General Hospital. "I was scared because I didn't want it to get too far into her system."

The copperhead, the only venomous snake among more than 15 species in the Washington metropolitan area, is commonly found in wooded and rocky areas, although one will sometimes creep into a subdivision. The snake is light tan with dark brown saddlebag markings along its back.

The snake is a lethargic one that bites only in rare instances, herpetologists said yesterday. Dr. Muriel Wolf, a specialist in snakebites who works at Children's Hospital, said that the copperhead's bite is the "least lethal" of all poisonous snakes in this country.

Symptoms of the copperhead bite include swelling, and nerve and tissue damage in the area of the bite. Copperhead bites are considered most serious when the victims are children or are allergic to the venom, Wolf said. One child she treated for a copperhead bite on the leg couldn't walk for six months, she said.

Another danger is the antivenin, made from the blood of horses injected with copperhead venom, which can cause swelling and a range of serious reactions, she said.

For most adults, the symptoms of a copperhead bite are "very much like a bee sting," said Dale Marcellini, the National Zoo's curator of herpetology. Although the copperhead is common in wooded areas ranging from Kansas to Georgia and north to Connecticut, deaths from copperhead bites are "very rare," Marcellini said.

"Copperheads get a bad rap," Marcellini said. "They're usually no problem at all."