The fire burning in southern Bannock County near Swan Lake grew to 1,200 acres Friday.
The Eastern Idaho Interagency Fire Center said the blaze, known as the Stockton Fire, is 25 percent contained.
There are no evacuations, but it the EIIFC said Thursday night it was threatening some structures.
Downey, BLM and Franklin County crews are fighting the fire. An airplane was dropping retardant, and a helicopter was at the scene.
The fire started around 2:30 p.m. Thursday and was started by farm machinery. It is burning mostly on grazing land.