The fire burning near the Blackfoot Reservoir grew by 100 acres, as of about 10:30 a.m. Friday.
The Fox Hill Fire started Thursday and is human-caused, the Eastern Idaho Interagency Fire Center said.
Structures are within one mile of the fire and are considered threatened until firefighters stop the blaze from spreading.
Four engines, one dozer, two single-engine air tankers, a hand crew and two helicopters are on the scene.
The fire is human caused.
The EIIFC said it expected to have the Fox Hill Fire contained by 6 p.m. Friday.