Anne Arundel County police called it a prank. The state Department of Natural Resources said it was a rare botanical phenomenon. The U.S. Environmental Protection Agency said it did not know what it was. And one of the discoverers of the mysterious glowing tree branch, which caused a stir from Upper Marlboro to Annapolis, said that "it freaked us out."

The mystery, which stumped police, fire departments and state agencies most of the day, began about 1 a.m. yesterday when six teen-agers from Lothian, Md., took a drive about a half-mile into a wooded area along a road from Lyons Creek Mobile Estates, where they live.

"We were partying," recalled Julius Jones, one of the youths. "I looked down on the ground and saw this thing glowing.

"It was shiny and smooth and cold, and we were all pretty shaken up," Jones said. "At first I thought it was radioactive."

The branch, said Jones and his friends, was emitting green light that could be seen 50 yards away, but the branch glowed only when parts of the bark were chipped away to expose the soft, shiny underlying wood. Although the tree and the area around it contained a number of glowing branches, it was something that the youths, who frequent the area, had never seen before, said George A. Chopic, one of those who found the branch.

The group took the branch to Jones' home about a half-mile away.

Fearing that they were dealing with a radioactive specimen, the group called the local police, fire and natural resources departments. "The cops showed up, but they didn't know what to say. They said to take it to the EPA," Jones said.

"So we it took it to Dash In," a local convenience store, Jones recalled, "and we tried to sell it, but no one wanted to touch it. Nobody wanted to fool with it."

By midmorning, a slew of local and state agencies had been called to examine the glowing branch, and initially there was much confusion over the source of the glow.

At first, Anne Arundel police reported that a prankster had applied glow-in-the-dark paint to the branch as a hoax, a police spokesman said.

"For the police, that was a good guess, but unfortunately it was wrong," said Herb Meade of the Department of Natural Resources. According to Meade and scientists who examined the branch, the cause of the glow is a rare fungal growth known as fox fire.

Fox fire, which scientists say was much more commonly found 40 to 50 years ago, can occur in diseased and rotting trees when the decomposition of the wood combined with the chemical elements in the fungus cause the wood to emit a phosphorescent glow.

"It's not hazardous, but it's unusual. We don't get too many reports of glowing trees around here," said Barbara MacLeod of the Maryland Forest and Park Service.

A forester who has worked in the area for more than 30 years said that this is the first time he has seen the fungal growth, but he added that it is not possible to document how rare the fungus has become.

Scientists said fox fire is not of great value except as a scientific specimen. Department of Natural Resources officials said they have no plan to study it further.