The Indian Run Fire in northwest Colorado's Routt County has reached 165 acres.
About 70 firefighters are battling the grass and brush fire near Hayden. While officials could not say how contained the blaze was, no structures were currently at risk.
The fire was first reported after a lightning storm moved through the area on Saturday, but officials have not given a cause for it.
The Indian Run Fire comes during a very slow wildfire season for Colorado.
Meanwhile, the Department of the Interior is working with the Bureau of Land Management and other federal agencies to prepare for the 2020 fire season.
The Trump administration has issued orders to reduce the risks and damage of wildfires, primarily in the West. Some of the plans include bulldozing potential fuel for fires, prescribed burns and an increased workforce with specialized equipment.
In Colorado, more than 27,000 acres will be part of the wildfire prevention plans.