Biscuit Basin to Fountain Flats
The Firehole flows north past Old Faithful, and the river through the Upper Geyser Basin is closed to fishing. It reopens where the Grand Loop Road crosses it at Biscuit Basin, about two miles north of Old Faithful, and the next ten miles down to Firehole Falls are the water everyone comes for: weedy glides, long riffles, hot springs draining in from Midway and Lower Geyser Basin, and pullouts and parking areas along the road. Fountain Flat Drive and the old freight road give access to the meadow water below Ojo Caliente, where bison are usually in the way.
Firehole Canyon
At Firehole Falls the river drops into a short canyon, and there is only about a mile of fishable water below the falls before it meets the Gibbon at Madison Junction to form the Madison. The one-way Firehole Canyon Drive leaves the main road just south of Madison Junction and reaches it. This canyon is upstream of the junction, not below it; it matters because migrating fish from Hebgen Lake can reach the canyon and because its salmonfly hatch, driven by the warm water, is one of the earliest in the region, usually the first week of June.
Brown and rainbow trout
Browns and rainbows share the river in good numbers, and the Firehole is not a big-fish river: Montana Angler puts the average at 8 to 12 inches with fish over 15 uncommon. What it offers is rising fish. Midges and blue-winged olives open the season, PMDs arrive in early June and the White Miller caddis, a big cream Nectopsyche, is the signature hatch of June and again of September. Brook trout live in the headwaters above Old Faithful, small water that is fun but not a destination.
Season and regulations
Guides like the Firehole around 300 to 400 cfs, and this page treats 250 to 450 cfs at the Madison Junction gauge as in range. Opening-week flows can be high and tea-colored but the river is rarely too muddy to fish. From roughly mid-June to Labor Day the thermal inflows push water temperatures near 80 degrees; the park closes the river when that happens and local shops do not fish it in summer regardless.
The Firehole down to Madison Junction opens May 1 and closes October 31, earlier than most park water. It is fly fishing only, barbless hooks, lead-free weight, no felt soles. Rainbows and browns are catch and release; up to five brook trout may be kept. Anglers 16 and older need a Yellowstone National Park fishing permit; state licenses are neither valid nor required inside the park. Check the current park fishing regulations and closures before you go.