Southeast of Bozeman, the Yellowstone River offers excellent flyfishing opportunities. This is a long river that is unparalleled in beauty. This 250 mile long freestone river is dominated by cutthroat trout upstream and brown trout downstream. Plus, there is a mix of rainbow trout which average around 15 inches.
Many people think this river starts at Yellowstone Lake, but do not forget there is 30 miles of headwaters above the lake, too. It is a very isolated stretch accessible by horseback or hiking. Definitely expect to find plenty of mosquitos and even some grizzlies seeking a free meal. The best time to hit this stretch is in August. You can expect to find only cutthroat trout in this very narrow stretch of river.
The Upper Yellowstone from the lake drainage to the park boundary stretches a surprising 115 miles while passing through two canyons with walls over 1,000 feet up. The best time on this section is mid-July through August.
The Middle Yellowstone runs from the park boundary near Gardiner over 103 miles down to Big Timber, Montana. This is probably the most well-known stretch with famous brown trout, and equally famous side creeks. This section can be fished year-round, if you are so inclined. From Gardiner to Livingston, the river is paralleled by SR89. Downstream of Livingston, the river is dominated by large brown trout and theere is good fishing all the way to Laurel (which is paralleled by SR90).
While fishing, keep an eye out for grizzlies, bison and elk.
Overall, Yellowstone River is the 9th most popular fly fishing river of all 21 fly fishing spots in Montana.