The experts say every serious fisherman goes through three phases in his life.

First, he seeks quantity. A good day is measured in numbers, pure and simple. Hence the glowing accounts of a trip on the Chesapeake where the crew quits early with bluefish tails poking from the tops of chock-full trash containers.

Next he goes for size. That accounts for the surprising number of seemingly normal humans who fork over $400 to charter a boat out of Ocean City on the off chance that a marlin will cross their blue-water paths that day.

Finally, responding to the shining beacon of complete failure of reason, he seeks the most difficult.

In my book, the spells mountain trout.

It is fortunate for the fisherman that he comes to this crossroads late in his angling career.Pursuit of trout requires the patience of a scholar, the athletic skill of n acrobat and complete submission to a mysterious aquatic subculture.

To what end?

"Every trout fisherman I have ever spoken to has told me the one angling experience he will never forget is catching his first trout on a dry fly," said Mike Snyder, a born-again mountaineer who opened the Fastwater Flyfishing School in Harman, W.Va., this spring.

Of course, it's in Snyder's interest to make flyfishing sound like the greatest thing since sunshine.

I said, "Show me." So he did.

A gullywasher had turned the creeks and rivers muddy brown the morning before I arrived. Snyder met the little twin-engine Beechcraft from Washington - he calls it the Jennings Randolph special - at the mountaintop airport in Elkins.

"We'll have a time finding clear water today," he said.

The Laurel Fork, which runs behind his farmhouse and blacksmith shop in Harman, was a murky torrent, unfishable on any terms.

The Shavers Fork was as bad, and so was the Red Fork. The mighty Cheat, which runs to the Ohio, was fouled by all its feeder streams.

Snyder figured the high-elevation headwaters might be better.

He pulled his ancient station wagon into an opening in the forest high in the Alleghenies, slipped on his chest-high waders and set off down a ridge, chewing on a twig from a black-birch tree.

At the bottom was Glady Fork, nestled between hillsides of bright green spring foliage. The water was tainted but not hopeless.

He picked a flat rock from the stream bottom and turned it over. A six-legged black creature crawled off in fright.

"Good," said Snyder. "Now look at this." He plucked a flybox from his vest, picked out a six-legged black creature made of fur and feathers and tied it on the line. He sent me off to fish, hanging over my shoulder and giving pointers.

A fish slurped up the submerged fly a few minutes later, but the novice student doing the fishing had forgotten a few key prerequisites for hooking and landing a fish. The trout swam off.

Two miles downstream, with an hour of daylight left, we turned back. Now Snyder switched to dry-fly fishing.We had turned upstream and could cast the fly ahead. With proper manipulation, it came back down in the current in a dead drift, coursing over rocks and eddies like a helpless, just-hatched mayfly.

"Look," Snyder shouted as we neared the road. He snatched the hat off his head and swiped at the air. Then he poked in the hat with a finger and pulled out a bug.

"March brown," he said, grinning. "They're starting to come off the water. This is it, man. A perfect mayfly hatch."

I looked around and, indeed the air was full of these thin, dainty bugs with upturned tails.

Snyder tied on an imitation and sent me off to a prosperous-looking pocket.

I cast upstream and watched the bug swirl back, adjusting line and rod tip so the bug floated just like the real thing.

Within minutes, I was wrapped up in it. The mayflies were popping off the water, trout were swirling the surface as they gobbled the bugs, the water was rushing around my waders and my imitation mayfly was in the middle of everything, just waiting to get picked out by a trout.

On about the 20th drift, it hit the pocket just right. We watched a swirl of silver as the trout rose to the fly and snatched it off the water. In a minute it was over. The trout - a 12-inch brown - was netted, unhooked and set free.

Snyder smiled. "That's it. Just like in the books."

Fly fishing is the hardest angling talent to learn. Snyder's school offers a good first step, plus free camping in his backyard in the mountains and fine country cooking from his wife's kitchen.

For information, write Fastwater Flyfishing School, P.O. Box 60, Harman, W.Va. 26297.

His is the only full-time fly-fishing school in the Mid-Atlantic states. Closer to home, Anglers Art in Georgetown can set up instruction packages for day trips to local streams.