HELENA FALLS - The North Fork fire between Great Falls and Helena had burned more than 300 acres by midday Tuesday.
Department of Natural Resources and Conservation spokesman John Grassy says officials believe lightning struck in the area on Saturday and Monday's wind storm caused it to flare up.
The wildfire is burning timber in steep and rugged terrain near Stickney Creek northeast of Craig in Lewis and Clark County just south of the Cascade County line.
The DNRC says two Hotshot crews, two 20-person fire crews, 15 engines and three agency helicopters are aiding in the firefighting efforts.
The fire was spotted at around 7 p.m. Monday. No structures are immediately threatened.