Larimer County says the fire is about three acres in size and it is burning in the Hewlett Gulch area Greyrock Meadows, north of Poudre Park. It is about one mile south-southeast of Grey Rock Mountain.
It began just before 1:30 p.m.
Larimer County Emergency Services is working with the U.S. Forest Service to fight the fire. One helicopter was launched for air support and made bucket drops.
About 15 to 20 firefighters responded and additional resources, some coming from the fire in Boulder's Lefthand Canyon, will be available on Tuesday.
No structures are in danger and fire crews plan to stay at the fire overnight.
The cause of the fire is not known.
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