Maybe the most easily accessible trout stream in the West, the Gallatin flows north out of Yellowstone National Park’s western edge and is closely paralleled much of the way by Highway 191, so pick a likely spot and wade in. And wading is the key here, boat fishing is prohibited much of its length. Also, be careful where you fish, if you are within the boundaries of Yellowstone NP you will need a park permit, outside the boundary requires a Montana fishing license.
The Gallatin is classic western flyfishing, with awesome scenery (much of “A River Runs Through It” was filmed on the Gallatin), cool fast water and PLENTY of fish. Westslope cutthroat, browns and rainbows are all present in the Gallatin.
Watch the water level, the river runs high and fast during snow melt and during high rains. Dry flies are the key here but check with one of many local guide shops to see what is hitting. I used Gallatin River Guides, on Highway 191 near Big Sky, gallatinriverguides.com and Grizzly Outfitters in Big Sky, grizzlyoutfitters.com.