A wildfire that started near Interstate 90 about two miles northwest of Superior had grown to an estimated 500 acres by Monday morning.

The West Mullan Fire started at about 5 p.m. Sunday and had burned about 150 acres by 8:30 p.m.

Lolo National Forest spokesman Boyd Hartwig says the fire led to the evacuation of about a dozen residences.

Fire crews are trying to keep the fire from moving east toward residences.

Two crews are fighting the fire with a dozen engines while two helicopters drop water. Hartwig says another helicopter is on the way and fire managers have requested two 20-member hotshot crews and three more helicopters.

The cause of the fire is under investigation.