No injuries have been reported.
Authorities didn't immediately know how many structures burned down. But at least some were houses, including four that belonged to firefighters battling the blaze, said Laura McConnell, a spokeswoman for the fire management team.
The fire started Monday morning in Four Mile Canyon northwest of Boulder, and erratic winds gusting to 45 mph spread the flames quickly, sometimes in two directions at once.
About 100 firefighters were on the scene, and the winds calmed enough by late afternoon that three aerial tankers could start dropping fire retardant before darkness.
Some crews remained at the fire through the night. Four more aerial tankers were requested to join the fight Tuesday morning.
At least a half-dozen roads in the area were closed, and a billowing, white plume of heavy smoke was visible for miles before sunset. County health officials advised residents to stay indoors if the smoky air became irritating.
The fire's cause was unknown.
Boulder County sheriff's Cmdr. Rick Brough said the fire moved quickly through difficult country.
"It's very rocky, hilly, mountainous terrain," he said.
One fire vehicle was destroyed in the blaze.