It started when a lightning strike hit a tree and it burst into fire.
``I heard it hit - it sounded like a sonic boom,'' said Julie Pratton, who lives near the Reed Point Elementary School.
The Whistle Creek fire was being fought by six rural fire departments, two helicopters dipping water out of the Yellowstone River and personnel from the Department of Natural Resources and Conservation.
Stillwater County spokeswoman Karen Tyra estimated the fire was at least 800 acres. She said it would be Thursday before officials could determine its extent.
About 30 head of cattle were quickly moved out of the fire's path as it burned up to the pavement of Interstate 90 and spotted into the median, officials said.
The Montana Highway Patrol said traffic was disrupted - then stopped - because smoke limited visibility through the evening. At times, one-lane traffic proceeded with visibility limited to 100 yards, the patrol said.
A shop at the farmstead of Joan and Bill Langford was damaged.
The Stillwater County Commission declared an emergency because of the extreme fire danger and the dangerous Whistle Creek fire, said Commissioner Maureen Davey.
Fire crews from Reed Point, Columbus, Park City, Absarokee and Rapelje fought the fire, and the Big Timber Fire Department attacked from the west side of the blaze.
Wind and lightning also started at least five grass fires and threatened six ranches in Chouteau County, in north-central Montana. One of them spread over more than 500 acres some five miles north of Fort Benton, officials said. Three farms and ranches were within one-eighth of a mile of the blaze.
Wind knocked over a power pole igniting a 200- to 300-acre fire near U.S. 87, to the south of the Missouri River community, said Sheriff Doug Williams.
He said there were numerous power outages resulting from downed utility poles, but there were no injuries and no buildings damaged.