A U.S. Department of Transportation spokesman said the cause is under investigation, but there's no immediate public safety threat from the leak. It occurred about 15 miles from the city of Lodge Grass.
The agency spokesman said Houston-based Phillips 66 Co. workers found the leak Wednesday after pressure dropped on the line.
The environmental impact remains uncertain. The DOT spokesman said no waterways were immediately impacted.
Officials from the Environmental Protection Administration and the Crow Tribe also are investigating.
The same pipeline broke twice in a week in 1997, spilling a combined 2,300 barrels of gasoline near Lodge Grass and Banner, Wyo.