It is hot enough to make fish sweat and fishermen fry. At 6:30 a.m., radio weather reports were already describing the day as "hot and sultry." By noon, the elm trees along the banks of the Potomac looked like weeping willows.

But standing thigh-deep in the middle of this wide river, with the breeze in our faces and relief just a belly flop away, the day feels absolutely fine.

"This is a lot better place to be than downtown Washington," says David Lee, a fishing guide who has invited me to escape the air-conditioned discomfort of the city to fish for smallmouth bass on the upper Potomac.

Lee has managed an escape of his own. Two summers ago, after printing up a brochure advertising himself as an expert, he began taking customers on float-fishing trips down a river he has fished for 25 years. Otherwise, Lee would be spending his summer in a classroom in Hagerstown, breathing chalk dust as he taught college students a beginning course in psychology.

"Right now, we'd probably be discussing medieval witchcraft and phrenology," said Lee, a 39-year-old professor of psychology at Hagerstown Junior College. "It's a good course, but I've already taught it 120 times."

With any fishing guide worth his fee, you get taken to where fish are supposed to be and encouraging advice on how to catch them. With Lee, you get a little river history, geology and abstract theory thrown in at no extra charge. Lee may be the only guide within 100 miles who can discuss Jungian precepts while tying a no-slip knot.

Lee is unusual for other reasons. While most people fish for bass with high-tech spinning tackle and powerful boats, Lee flycasts for the fish from a modified, motorless canoe.

"I have nothing against any other kind of fishing," said Lee, paddling his square-backed "scanoe" through a set of riffles. "I can just catch more fish by fly fishing than any other way."

There are easier ways to catch fish than using a nine-foot rod like a whip handle to drop a tiny, almost weightless imitation of a bug or fly onto the surface of water 20 feet away. To be good at it, you should have the hands of a pianist, the knowledge of an entomologist and the patience of a cat. The demands of the sport, said Lee, are precisely what keep enthusiasts as hooked as any fish they ever catch.

"Fly fishing has layers of complexity, just like an onion," said Lee, who once, for the love of fishing, lived for a year in a trailer beside the Potomac without electricity or running water. "Most people I take out like things to be complex."

Lee has written a book to make initiation into the sport a little easier. "Fly Fishing: a Beginner's Guide," published in March by Prentice-Hall, is a 190-page book that mixes quotations from Roman historians with practical advice on how to sharpen hooks. The book places Lee in the company of a zillion other authors who have written about fly fishing in the last few centuries. But his book is selling.

"It's already into its second printing," said Lee, who has gotten rave reviews from half a dozen newspapers, including the New York Times. "I don't consider myself a great fisherman, but I am a better-than-average teacher."

The upper Potomac is a lovely place to do both. In the past 10 years, as the small towns along the river in West Virginia and Maryland have built sewage treatment plants, the water has become an attractive home for bass.

The seven miles we are paddling, from Scrabble, W.Va., to Snyder's Landing in Maryland, probably looks much as it did when George Washington envisioned it as a major trade route.

On the Maryland side of the river, the bank is protected from development by the C&0 Canal that runs beside it. Much of the land on the West Virginia side is too steep to build on. As a result, the waterfront acreage is still mostly green, tangled and occupied by beaver, deer and a hundred varieties of birds. Between fishing and floating, I see green heron, wood ducks, vultures and a water snake. Meanwhile, Lee is watching the small swallows that dart just inches over the water, snatching up bugs before the fish can leap for them. By watching the flight of the swallows, Lee can tell what kind of bug they are catching. Since the local fish are feeding on the same bug, Lee changes his lure to imitate the same.

I stick with a yellow popping bug, a square piece of yellow plastic, half the size of a sugar cube that covers the top part of a small hook. Yellow and black tail feathers and white rubber legs complete the deception. Once the bug has been cast where it should be, or relatively close in my case, it is jiggled and pulled to imitate a bug doing the breaststroke.

For 30 casts in a row, the jiggling is for naught. Then, with a just-audible slurping sound, it disappears into the mouth of a fish. The thrill is the same that any kid gets seeing a bobber pulled under. But with bass, if you allow yourself even a second to enjoy the sight before yanking the line to set the hook, the fish will take a taste of its meal and spit it out.

By the end of the day, we have caught and released about 15 fish. Another dozen have gotten away. On a normal fly-fishing day on the Potomac, said Lee, he and a customer would catch about 30 fish. But neither he nor I are disappointed. The sun has disappeared behind a wall of rain clouds that will soon cool the highway home.

"One thing people always insist on in their sport," said Lee, sounding more like a professor than a guide, "is failure."