Thunder Bay Fire Rescue crews were called out overnight to a house fire on the city's north side.
Fire had broken out in the basement of 241 Blucher St. at about 2 a.m. Monday.
All residents were out of the house before firefighters arrived, having been alerted by smoke alarms.
Firefighters were able to extinguish the flames quickly, but not before half of the duplex suffered fire and extensive smoke damage.
A total of 22 firefighters, five pumpers and one command unit attended the blaze, according to a news release from Thunder Bay Fire Service.
The cause of the fire remains under investigation.