A fire about eight miles northwest of Lincoln was estimated at 550 acres Monday.
The Klondike Fire was first reported around 1:30 p.m. Sunday and spread quickly because of windy conditions, according to Helena National Forest spokeswoman Kathy Bushnell.
The cause has not yet been identified, and the fire is not threatening any structures.
An area including Beaver Creek Road between Pine Grove Campground and Arrastra Creek Trailhead and all of Forest Service Road 607 has been closed.
The Helena Hotshots, Lincoln fire crew and Cherokee Hotshots responded and were joined by a fourth crew on Monday, Bushnell said. Four air tankers dropped retardant on the fire throughout Sunday afternoon and evening, and helicopters and air tankers continued to assist ground crews Monday.
Bushnell said crews hit the fire early Monday morning to get a start on the fire during the cool weather and focused on the southern flank through the day.
“The hike up to the fire is somewhat difficult -- through heavy dense brush and steep terrain,” Bushnell wrote in an email.
The fire showed low to moderate activity through Monday morning.