Ketchum, Idaho: It was snowing at the Big Wood River. With soft powder on the banks, nearly barren cottonwoods framing the water and the air a chilly 32 degrees, you might have assumed that it was too cold to fish. But Scott Schnebly stood in the water, casting his zebra midge into the near-freezing stream, and caught a rainbow trout.

Mr. Schnebly, who owns Lost River Outfitters, has been fishing these waters for 30 years, and he has come to love this time of year. Brutally cold conditions mean hungry fish and fewer people trying to catch them. On a recent trip, we fished for two days, and we saw only one other person willing to endure the 41-degree water.

The winter months bring solitude to Western rivers because most fly fishers have packed away their gear. For Mr. Schnebly, and a small - some might say insane - group of anglers, December marks the opening of the winter fishing season, and a pretty good one at that. On winter days, usually from noon to 4 p.m., bugs about the size of a baby’s toenail will hatch atop the water, and trout and whitefish will rise to feed on them.

Of course, winter fishing is not entirely unknown. Across the country, ice fishermen sit on frozen lakes, drilling holes and dropping lines. But fly fishing is different, if for no other reason than that anglers stand in running water that can chill the body to its core.

Although winter fly fishing cannot yet be called a popular sport, there is growing interest as more and more guides, from Yellowstone to Taos, offer winter fly fishing trips, often near ski areas. John Duty, owner of Bucking Rainbow Outfitters in Steamboat Springs, Colo., takes clients by snowmobile to find remote tail waters, stretches below dams that stay free of ice because of the warm, consistent flow from the reservoir.

Recently, I asked Mr. Schnebly, 54, who has guided Byron R. White, the former Supreme Court justice, and the actress Jamie Lee Curtis (but not together), to take me on a cold-weather trip on his home waters. I had cold-weather fly fished a couple of times in Colorado, but the only thing I caught was a cold. Mr. Schnebly told me to pack polarized glasses, fingerless gloves, noncotton long underwear, fleece pants, polyester-blend socks, a turtleneck, fleece jacket and a waterproof jacket. He provided the waders, a rod and reel, flies, ice-off paste and a lot of local knowledge.

The night before our excursion, Mr. Schnebly told fish stories in his kitchen as he made paella with pheasant, chukar, venison, halibut, clams and garden-grown peppers and tomatoes. As he breasted three mallards, he said, “Winter fishing can be absolutely beautiful with the bright blue skies against the snow white background and, of course, the solitude.”

He was right about the solitude. Our first day was cold and gray with snow flurries. Early on a blustery Sunday morning, we put on five layers of clothing, plus our waders, and met Lynn Dominick, 62, a retired first-grade teacher who wanted to improve her fly casting. The Big Wood River, which cuts through the Wood River Valley of central Idaho, runs a short distance from her house, so we parked in her driveway and walked to and into the water. We were thigh deep in the gushing water. Ice fishing might be the popular symbol for fanatical fishermen who will endure sub-zero temperatures just to fish, but winter fly fishermen could do well in any suffering contest because even with waders made with Gore-Tex, standing waist-deep in a river numbs the legs.

Along the bank, near a fallen cottonwood tree, we could see trout rising to the surface and feeding. Mr. Schnebly identified little insects, called midges, floating by and looked in his bait box trying to match them (Fly-fishing depends on getting the fish to mistake one’s artificial fly for the insects they’re feeding on). He tied a man-made midge on the end of the line with another, larger dry fly a foot away as an “indicator” - to let him know if a fish has taken the bait. Using a nine-foot graphite rod, Mr. Schnebly tossed some casts. Ms. Dominick watched, mesmerized by his timing, the line arching back like a sling shot, and as it went forward the bait pirouetted on the water surface and then sank. Soon Mr. Schnebly caught an 11-inch rainbow.

For the next three or four hours, Ms. Dominick practiced her casting, trying to present the fly in a similar fashion. Mallards flew by, water dippers dipped their beaks, and Mr. Schnebly’s dogs, Max, a wire-haired griffon, and Bart, a golden retriever, played among the cottonwoods. Mr. Schnebly explained that cold conditions bring subtle differences in fish behavior: they don’t move as much, they spend more time in schools, and they mostly eat smaller insects because the river temperature is too cold to produce anything larger. This means that it can be slightly more difficult to catch fish because the accuracy of the cast becomes paramount, and because smaller flies which are lighter, are more difficult to cast and present effectively.

“It’s about as interesting and as difficult as it gets,” Mr. Schnebly said, as Ms. Dominick stood 40 feet away from a pool and tried to land a fly in an area as large as a cocktail tray. After numerous casts - some accurate, some not - the fish still would not take the artificial fly. Irritatingly, they kept eating the real bugs. “Maybe they are feeding right when the midge breaks the surface, and we can’t mimic that,” he said, as he tied a slightly different fly to the end of the line. Mr. Schnebly was perplexed.

So after an hour, we shuffled up stream. It started snowing, and our feet became cold. But Mr. Schnebly and Ms. Dominick, whose son is a fishing guide in Wyoming, were unfazed by the conditions(I couldn’t feel my left foot, which made me trip on the river rocks). Mr. Schnebly decided to put on a larger and heavier streamer fly made with the feathers of a ringed-necked pheasant. After four casts, Ms. Dominick landed a 10-inch rainbow.

AT 4 p.m. Ms. Dominick decided to go home for some soup(She planned to return later). In the fading daylight, we drove two miles up stream and arrived at the Red Cliffs pool. We were surrounded by cottonwoods and basalt outcrops. It started to snow again, and we were losing light. Using a zebra midge, Mr. Schnebly caught a 17-inch rainbow trout. But after an hour, even Mr. Schnebly, started to give up on the day, shamed that we had only caught four fish.

“Have the fish grown too cold? Is it just too late in the day for them to feed?” he muttered. The temperature was dropping, and yet we kept casting. Mr. Schnebly patiently reminded me to stop jerking my arm during my casts, which had none of the beauty, and to this point effectiveness, of his perfect strikes. Using a larger fly, a streamer called the Philo Betto (named for Clint Eastwood’s character in “Any Which Way But Loose”; yes Eastwood fans, the character’s name was really Philo Beddoe), which mimics the Wood River sculpin, I presented the bait. Mr. Schnebly implored me to “mend, mend,” which meant to let the flow of the river take the bait in a natural way, so it would mimic a bottom dwelling fish, and then to “strip, strip,” which meant pulling the line to look like the fish’s darting motion. At 5:30 p.m., in gill-chilling weather, I landed a 15-inch trout. We returned to his truck and packed up, returning to his store 20 minutes later for Cognac in paper cups and fish stories.

The next day we fished on a part of the Big Wood, called the Hailey Riverfront. We were about 11 miles south of what had been Hemingway’s house. It had snowed all night so the banks were covered in several inches of powder. Mr. Schnebly’s earlier prophecy of a winter wonderland came true: it was warm for an hour or so, with a blue sky.

“I don’t go on the Big Wood in the summer anymore because there are too many people - I can’t get on it,” said Mr. Schnebly, who said he once fished in conditions so cold that his reel wouldn’t work.

Once again, we did not see anyone on the river, but a river ecosystem is hardly quiet, even in the winter, with the water flowing over boulders, the riffles and the quieter pools. We stood in the middle of the river, and I cast near the bank. But nothing happened as we walked a couple hundred yards down the river. Finally, after scratching his hat a few times, Mr. Schnebly decided to cast up stream in the riffles, trying to mimic a fly washing off the rocks. “This presentation looks more natural to the fish,” he said. The wind blew through the willows and cottonwood leaves plopped in the water.

Suddenly, Mr. Schnebly’s line grew tight. He caught an 18-inch Rocky Mountain white fish. He smiled as he brought it in. He had once again solved the mystery of the river and the fish within it. Minutes later he caught a rainbow trout, and then another, and another, releasing them back into the river for the next winter fisherman to catch.