Three fire engines from the county town responded to reports of smoke issuing from a flat that was "well alight" on Trinity Street in Taunton at 9.33pm on Saturday.
An Avon and Somerset Police spokesman has confirmed that a woman in her 50s suffered serious but non life-threatening injuries, and is currently being treated at Musgrove Park Hospital. She may later be transferred to Frenchay Hospital in Bristol.
A woman suffering from smoke inhalation was treated by ambulance teams, as firefighters set to work extinguishing the fire with four breathing apparatus and two jets.
Police are investigating the incident.