It broke out before 5:00 a.m. Wednesday in the unit block of Market Street in Bangor.
Images from Chopper 6 HD shown on Action News showed some flames and smoke coming from the building more than 2 hours later.
There have been no reports of injuries.
The American Red Cross is assisting 12 residents - 7 adults and 5 children - displaced by the fire.
The cause of the blaze is under investigation.