I asked Ben Schley, who has fly-fished the best rivers in the world from Alaska to New Zealand in the company of such luminaries as Howard Hughes and Presidents Carter and Eisenhower, to name his favorite fishing place.

"The Potomac," he said, without hesitation.

Schley grew up on the river near Shepherdstown, W.Va., where he was born 70 years ago. He's moved around since, and for years ran the fish hatchery program for the U.S. Fish and Wildlife Service, which gave him access to great fishing everywhere. But it's the Potomac he came back to.

Last weekend he and Mark Kovach, a professional fishing guide, invited me to fish the river from Kovach's rubber raft, rigged so he can row clients from hot spot to hot spot.

The aim of these two was to show off the Potomac's phenomenal white miller hatch, an explosion of mayflies that erupts at dusk each day for about two weeks on the 20-mile stretch below Harpers Ferry.

I'd heard about the white millers but never experienced the hatch. "It's a snowstorm," people said. "The bugs are so thick you worry about inhaling them." The hatch also is supposed to produce great fishing as smallmouth bass rise to slurp the inch-long insects when they emerge.

I went along, thinking that, like most things fishing-related, the white millers probably weren't as remarkable as everyone said. Wrong again, negative nose.

At about 8:45 p.m., after a pleasant afternoon of catching smallmouths and red-breasted sunfish, we watched the first white millers pop off the water just above the Route 340 bridge. On the Virginia shore, a campfire glowed.

Kovach handed out small flashlights for seeing flies and lures in the dark but warned that when the hatch came on, any light was an invitation to a bug-deluge. He didn't say, "Let it begin," but it did.

Suddenly it was a silent blizzard on a warm night. There may have been a million bugs, or maybe a billion. It was a wall of moving white specks on a black canvas, like driven snow borne horizontally on a hard wind. But there was no wind here, just tiny life on a reproductive kamikaze mission.

Mostly they flew upstream, presumably to drop their eggs where they would drift back in the current to the home place. There were so many flies, they flew in your ears and down your shirt. The ones close enough to inspect were impossibly delicate, pure white, with narrow bodies curved like a scimitar, rising at the tail into two long forks, and with tall, milky, translucent wings.

But look fast. They don't stay long. The white millers' Latin name is Ephoron leukon, which means "lasting but a day." For a year they burrow in the mud and sand of the stream bottom as nymphs. Then, for two hours, they emerge as beautiful aerial acrobats that molt once on the fly, mate, deposit their eggs and die.

The white miller thus is a classic example of the mayfly order, Ephemerata, which means short-lived. Mayfly hatches are common, but very few places sport a hatch of the abundance of the Potomac's, and no other mayfly consummates its brief life of flight quite so quickly.

To tell the truth, the bugs were so plentiful on the Potomac the fishing became awful. No lure or fly interested the bass during the hatch -- there was too much competition from real food, the bugs.

In the dark you could hear the bass slurping, but how to trick a beast already gorging itself? A bass could lie in one spot, tip up every 20 seconds and take mouthful without moving an inch.

"The only way to approach these fish," said Schley, "is as you would a wary old brown trout. Work to the rises. Sneak up."

Which is hard to do in a big boat sliding downstream with the current in the dark. It was halfheartedly that I fished, anyway, so arresting was the sight of a moonlit mayfly deluge.

After a while, I noticed that Schley and Kovach weren't attending to duty, either. We three just sat in the boat, awestruck, until it was over.

The white miller hatch has been in high gear for about a week, which means at least another week to go. The river around Brunswick is wide and slow-moving, perfect for canoes and johnboats, but please wear a lifejacket.

The hatch is strictly at dusk, but fishing can be good in the area all day. Best lures: White Mister Twister; black and chartreuse jig 'n' pig; small crawfish crankbait; white spinnerbait. Best flies: Muddler minnow, white or chartreuse streamers, crawfish imitations, poppers.

Kovach, who lives in Silver Spring, charges $95 a day per person and carries two anglers. The fee includes a huge, splendid lunch and a fishing day that stretches over 14 hours, if you can stand it.