The lightning-caused Tyndall Meadow fire in the Boise National Forest spread to 50 acres by Friday, but "hotshot" crews from Oregon and Utah were on their way.
Frank Carroll, a spokesman for the Boise National Forest, said 110 firefighters and a dozen smoke jumpers continued to battle the blaze for the third day in the central Idaho mountains about 25 miles east of Cascade.
He said 200 firefighters would be on the blaze by Friday night.
Although Carroll said the fire was reported to be 30 percent contained, he said that figure may likely be reduced when another assessment is made later Friday. Meanwhile, there was no estimated time for containment or control.
Two 20-member hotshot crews from Redmond, Ore., and Utah were called to the fire Friday.