About 50 firefighters responded to a property on the 600 block of Pasadena Avenue around 11:05 a.m.
Pasadena Fire Department spokeswoman Lisa Derderian says three units were declared a total loss. A house located in the front of the property also suffered significant damage.
Most residents were home, but were able to escape. There were no immediate reports of injuries.
Derderian says firefighters were able to rescue a dog suffering from smoke inhalation. The Pasadena Humane Society transported the dog to a local animal shelter. The dog is expected to be OK.
A resident's cat remains unaccounted for. A second cat in an adjacent unit died in the fire.
Residents were being allowed to retrieve personal belongings.
The cause of the fire remains under investigation.