The fire broke out around 5 a.m. on the 100 block of Main Street.
Flames burned through the second floor.
Witnesses said that two people jumped from windows to escape the flames.
Darby Fire Company No. 1 Captain Jimmy Watts said three adults were able to escape before firefighters arrived.
Watts said the house in the middle of the row is split into two apartments, with one occupying the first floor and the other occupying the second floor.
Apartments that are next door to the fire might have sustained smoke damage, but Watts said that still needs to be determined.
At least 15 fire departments responded to help extinguish the blaze.
The Red Cross is assisting families affected by the fire.
The Fire Marshal is on the scene, beginning an investigation into the cause of the fire.