If I had to choose one weekend of the year to go trout fishing, I'd choose this one. Here's why and here are the choices.

In Pennsylvania, the most spectacular and charming trout fishing of the year is about to occur. The reason: ephemerella gutulatta. Latin aside, gutulatta is a mayfly commonly known to fly fishermen as the "green drake."

To adent fly fishermen green drakes mean many things. For sheer irresistibility, I've heard them compared to blackjack, good whiskey and easy company. They are simply the phenomenon of trout fishing. When the drakes hatch off the water - they're aquatic insects - their evening swarms are often so thick that it appears as a blizzard in June. Because they are the largest of the eastern mayflies (about 2' long), they are one of the few species that will consistently provoke wary lunker trout off the bottom to feed on surface flies.

My first choice for a stream would be Penns Creek near Poe Paddy State Park east of State College, Pa. Other good streams are Fishing Creek north of State College and Spruce Creek south of State College. In case you're wondering why green drakes hang around State College, they aren't really after bacherlors' degrees - it's the rich limestone water that accommodates them. In Maryland, the best drake water is the Savage River in the western part of the state. Big Hunting Creek north of Frederick also gets a fair hatch of drakes.

The green drake hatch in brief, providing only about four days of furious fishing on any given stretch of a river. The hatching of the insects, and therefore the best fishing, will move progressively upstream as the water warms.

The best dry fly fishing is from 7 p.m. until dark, when the heaviest hatching and feeding occurs. During the day fish a sunken nymph in the lack water behind big boulders or in the pools where silt and sediment, in which the nymphs live, collects.Try to catch the initial day or two of the hatch when the trout will be most ravenous, bold and foolish with the sudden appearance of the big flies.

Green drakes lead a Cinderella life: For three years the brown larvae with furry gills, three tails and tusk-like mandibles live burrowing in the silt. Then, at this time of year, the larvae swim and twitch (so twitch a nymph pattern) their way to the surface to split their shuck and emerge as delicate cream-and-black-flecked creatures with a faint olive cast and speckled wings like cut glass. Their mouths have atrophied; they live and fly in beauty for only three days to mate, deposit eggs and fall spent on the river where their bodies become food for the trout. A spectacular phenomenon.

Because of their brief appearance, it's best to check hatching activity by phone. For streams near the State College, call Fly Fisher's Paradise at 814/234-4189. Potomac Valley Fly Fishers will sponsor a trip to the Savage River this weekend; call Monroe Mizel at 946-2900.