When they told Army Sgt. Carlos Sellers he was being reassigned from Fort Benning, alongside one of the best bass lakes in the country, to Fort Belvoir on the Potomac, he sold his bass boat.

Actually, he sold his good bass boat, keeping an old one just in case. He'd been stationed at Belvoir briefly in 1977 and knew there were a few bass in the Potomac, but he didn't expect much. He reckoned it a big step down from Benning's Lake Eufaula.

So what happened when Sellers took his old bass boat out on the Potomac and went fishing for the first time? He caught 27 largemouth bass, the largest 5 1/2 pounds. Next time out he and a friend caught 21, the largest four pounds. That was more than a year ago. He's been catching bass and kicking himself for selling his good boat ever since.

Sellers became an instant expert on Potomac bass fishing, which surprised him as much as anybody.

He's baffled by the lack of competition. Last week he and I piled the coves and backwaters near Alexandria for eight hours, jerking fat keeper bass out of the water as we went, and saw not another angler at play. "I don't know why people don't fish it," he said in his soft Georgia drawl.

Sellers is no newcomer to the bass scene. He has been selected each of the last four years to compete in the Bass Anglers Sportsman Society's national federation classic. This year he made the six-member Virginia team by finishing sixth at the qualifying tournament last month at Buggs Island Lake.

He takes bass fishing seriously, which is why he figured he'd basically be giving it up. "When I came here I thought the fishing wouldn't be anywhere near what I was used to. I didn't anticipate doing much fishing, based on what I'd seen in 1977. But I've since changed my mind. The bass fishing is excellent. I'm amazed at the way the river's been cleaned up.

"Considering the factor of the urban nature of the river, it is an excellent fishing river. Even on an overall scale (judged against less citified rivers) it's good."

He said the proof of the river's health is in the "consistency of production of largemouth of good size," adding, "these fish are extremely healthy and extremely fat."

Oddly, when Sellers pulls out of his home port at Belvoir, just south of Mount vernon, he heads upriver rather than down. Although he concedes the river's best fishing lies south in creeks like Aquia and Potomac, many of his favorite spots are upriver within sight of the Woodrow Wilson Bridge, including some above the bridge just across from the Alexandria waterfront.

The run to Alexandria is much shorter and uses less fuel. Except for special occasions, this is where he fishes. There are some rural surprises, even this close to the city.

During our foray last week we encountered, in addition to a half-dozen good-sized bass, a flock of Canada geese including many goslings hatched this May, blue herons, a beaver hut, wood ducks and mallards, striped bass, catfish, turtles, two owls (heard, not seen) and perch, we think. The perch were too small to take the big bass hooks, but the taps when they bit felt identifiably perchy.

Sellers' army specialty is terrain analysis and cartography, which means he knows the lay of the land. He has raised to high art the practice of fishing underwater geographical phenomena like dropoffs, channel edges and shallow ledges near deep holes.

His stops along the river come complete with explanations of the specific nature of the bass holding grounds, as in "this is where they dumped some old concrete building material. You'll get your lure hung up on the steel reinforcing rods in the rubble, but the bass lie in there. Toss behind the rubble and when you drag the bait over top of it and it falls, that's when he'll hit."

All this is probably true but it takes him, not me, to prove it. I'm the kind of bass fisherman who tears them up when they decide to be torn up. When the fish are being contrary it takes Sellers' practiced touch to get them to bite.

He wound up catching all the bass on a motor-oil colored, glitter-flecked six-inch plastic worm while I flailed the water with Sellers' personally designed "Waterwitch" spinnerbait, normally his favorite lure.

All I caught were five of the biggest catfish of an illustrious catfishing career. The big national bass organization being B.A.S.S., Sellers said he'd recommend me for membership in a brand new outfit -- C.A.S.S., the Catfish Anglers Sportsman Society.

Sellers is the premier member of a young Fairfax club called Bass Busters. His pround colleagues in that organization called to notify me of the existence of this "hot new fisherman."

No question. He's hot.

Sellers is not secretive about his Potomac spots. He favors the Virginia shore of the river just below Belle Haven Marina, fishing around huge beds of lily pads and against the shoreline; Swan Creek near Tantallon Marina on the Maryland side, and several of the pools and backwaters on the Maryland/D.C. side just upstream of Wilson Bridge.

We found the best concentrations of bass in just such a backwater, where we fished in solitude to the constant hum and roar of commuter traffic blasting down Rte. 295, 100 yards away behind a bank of trees.

Weird, but productive.