The Monster of the Potomac - not Congress, but the river's version of the Loch Ness Monster - has been sighted in recent weeks near the confluence of the Potomac and the Chesapeake Bay, according to about 15 persons.

The witnesses, who said they have seen the creature three times, describe it as about 25 feet long and grayish. They said it undulates as it moves quickly through the water. And one of them even fired at rifle shot at it.

Robert Norris, who should know which creatures inhabit the river and which do not, said he would "guess" it was porpoise. "I've lived on the river all my life - 65 years," said the executive secretary of the Potomac River Fisheries Commission, and I have never heard of anything like this being reported.

"A whale got into the James River about 10 years ago," Norris added. "but the size they describe doesn't sould like a whale."

It was not a porpoise school in the eyes of Ann Kyker, who lives in a riverside home in Northumberland County with her husband Donald, a retired CIA employe.

"I know what porpoise look like. It wa just subsurface with its head barely up. Rather than seeing the whole body you had a feeling of the whole body," she said.

Donald Kyker, said he saw the monster in late July as it passed about 50 yards from shore.

"My first thought was that it was a sea otter followed by its family, he said. "But, the leader had a head and no one else did.

"My second thought was that it was a practical joke, a man-made object being towed, but there was nothing around to tow it."

Ann Kyker said she did not know until she read the local weekly paper that another family had reported a creature of the same description a week before the Kykers' sighting.

According to UPI, Mary I. Lewis, 23, said she had seen four or five serpents with heads "the size of the two fists and necks raised three feet above the water" June 27.

The Kykers, uncertain of what they had seen, called their weekender neighbors, the Smoots from Fails Church, and alterted them to the monster' approach.

Myrtle Smoot said yesterday that the family had seen four creatures and that her husband had shot one of them. Mrs. Smoot said they saw the big one, then three smaller ones, five to 20 feet long.

Howard Smoot, a 55-year-old sales representative for Wolverine Brass Works who has been going to the lower Potomac since he was 9, said, "When I hit it with the .22 rifle, the forward three or four feet reared up and then they all sounded. We went out in the rowboat but there was no sign of it," he said.

At the Virginia Institute of Marine Science in Gloucester, meanwhile, Dr. John V. Merriner said, "The trial is cold now, but we will try to track it anyway." He said he would question people who had seen it.

At Maryland's Chesapeake Bay Biological Laboratory, an official said. "There have been eels that big reported on the Australian barrier reef, but nothing comes to mind that would fit that description here. You got me. Id like to see it."