It was first reported at about 2 am Friday at the "Peaches and Queens" gym on Academy near 7th Street.
The battalion chief says the fire appears to have started in the attic of the building.
A second alarm was called in to keep the flames from spreading to adjacent buildings.
Firefighters from Fresno County and Reedley arrived moments later to assist, bringing in a ladder truck to take on the fire from above.
Academy was closed between 7th and 8th streets for crews on scene.