Black smoke was reported coming from an apartment at 400 91st St. at 9:40 a.m.
Fire crews were able to access the apartment and extinguish the fire before it spread to other apartments.
No one was injured, but three adults and one pet were displaced from the apartment.
The fire remains under investigation. The North County Fire Authority is reminding residents to have working smoke detectors and a fire extinguisher in the home at all times.