The surest place to catch fish in North America may be a rock-lined, tree-shaded pool of clear mountain water, three hours from Washington and so thick with rainbow trout they fight one another to jump on your hook.

If that sounds like a fish story, it is not. If it sounds too good to be truly sporting, it certainly is.

"This is sort of like taking a test where, for a fee, you get to look at the answer sheet," concedes Worden Robinson. He is the owner of Nethers Mill Trout Pond, a picturesque home to hatchery-raised trout that are kept healthy, if slightly hungry, on Purina Trout Chow until someone with a corn-coated hook can pull them out.

You need no license to fish at Nethers Mill Pond. Unlike almost every other trout fishing spot in the Western Hemisphere, there is no limit.

There is, however, one catch. You have to keep all the trout you land and pay 22 cents an inch for each fish you keep. Leave your hook in the water too long and you can reel in a hefty bill.

"Trout is pretty high in the store, anyway. At least here you get the fun of fishing, too," said James Beachy, a red-bearded carpenter from Madison, Va., who caught 10 trout last weekend in half an hour.

To fishing purists, and there are none more pure than trout anglers, Robinson's operation is close to sacrilege. The trout, which in its native state is among the wiliest of freshwater fish, is demeaned, they argue, by a life of captivity.

Ironically, one of the bastions of trout-fishing tradition, the Angler's Club, shares the Hughes River with Nethers Mill Pond. Earlier this year, a member of that 40-year-old club compared trout fishing at Robinson's to lion hunting in a city zoo.

But Robinson says he is not trying to hook the dry-fly aficionado. He is after people who like the taste of trout but have neither the time nor expertise to fool them with hand-tied bits of feather and fur. He is particularly trying to snag the interest of kids.

"I've turned a lot of kids on to fishing here," says Robinson. A former potter, mechanical engineer and Peace Corps volunteer, he owns the pond, a house and barn and a 195-year-old corn mill that he has converted into a crafts shop.

Here is a place to take a kid who is old enough to hold a fishing rod, but too young to stay with it very long. At Robinson's, a 6-year-old like Jordan Mayor of Leesburg, Va., can feel the tug of his first fish before he feels the boredom of waiting for it.

"Ooh, it's slimy," he said, agreeing to rub his finger along the length of the fish after some coaxing.

While Jordan stared solemnly at his fish, his 11-year-old sister Jessica was busy pulling in her first. She wore a long-billed purple cap and a smile bright enough to make her 13-inch rainbow look pale.

On the other shore, 2 1/2-year-old twins shared a rod and a fascination for the fish you could see in battalion strength swimming in the water.

"Here, fish, fish," said Ryan Diamond of Silver Spring, Md., as he waved his cane rod above the water like he was trying to swat flies.

With a bit of help, Ryan and sister Lea each pulled in a fish. Ryan squealed at the sight of his like he was being tickled. Lea took a cautious step back, then moved forward open-mouthed to watch the fish unhooked.

"The question most kids ask is, 'Are you going to kill it?' " said Robinson, as he cut open Jessica's fish to show her its heart and liver. "I say you can't eat it unless you kill it."

The naivete of children who fish the pond is not surprising to Robinson. What does confound him is the attitude of some adults. Because his pond is beside a narrow road that leads to Old Rag Mountain, Robinson expected city folk to stop on their way home from this popular hiking area to catch a fish supper.

"That hasn't happened much. These fish scare the hell out of them. It's too close to the food chain. They don't want eggs still warm from the chicken or fish straight from the water. They want the food processed."

Robinson does almost everything but process fish. He provides cane pole rods and bait, cleans the trout and packs them in ice for no charge. Because he doesn't make any money unless you catch fish, he also offers his best advice on snagging them.

Before Robinson began his operation last fall, he thought he had hit paydirt. "When I first got the idea for this, I came home and had two bourbons and thought I was going to be a millionaire. I'd never seen so much money. Next morning I called the fish people and I was broke again," said Robinson, measuring another mess of rainbows. "You don't get rich doing this . . . but you do get to eat a lot of trout."