To trout fishermen, Montana's Madison River is an almost-holy place, a shrine to the delicate sport of fly-fishing. Every year, tens of thousands of anglers descend on the river, which tumbles out of Yellowstone National Park and runs for more than 100 miles in a picturesque valley between the Tobacco Root and Madison ranges to Three Forks, where it joins with the Gallatin and Jefferson rivers to form the mighty Missouri.

Cold and mineral rich, the Madison boasts one of the nation's best wild trout fisheries--for every mile of river there are several thousand brown and rainbow trout, none of them hatchery-bred.

But in the last few years there has been a precipitous decline in the rainbow population on the upper Madison above Ennis, Mont. A few days before Christmas, officials from the Montana Department of Fish, Wildlife and Parks announced they had found a likely culprit: a deadly parasitic disease for which there is no known cure.

The discovery of what is known as whirling disease among the Madison's rainbows has alarmed fishermen, conservationists and fly-fishing guides who revere the storied river and depend on it for their livelihood. They fear not just for the Madison, but for the rivers to which it is connected as a principal tributary of the Missouri.

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"Through carelessness of some kind or another, one of America's premier trout fisheries has been badly trashed," said Pete Rafle, a spokesman for the national conservation group Trout Unlimited. "There are outfitters and shop owners literally weeping over this."

"Potentially, it's a disaster," agreed Dave Kumlein, a Bozeman, Mont., fishing guide. "If it moves throughout the Missouri River drainage, you've covered some of the best trout waters in the world. If it moves upstream, as it can, all the great trout rivers in Montana east of the Continental Divide could be affected--the Big Hole, the Gallatin, the Jefferson, the Beaverhead, the Ruby. It's a who's who list of world-class trout streams."

Whirling disease--named for the characteristic tail-chasing of young fish whose cartilage is attacked by parasitic protozoa--is a relatively new phenomenon in the United States. Accidentally introduced into the country in 1956, whirling disease has been detected in 18 states and has been associated mostly with hatcheries.