A firefighter was taken to hospital with heat exhaustion after tackling a fire at a disused building in Worthing, which is believed to be arson.
West Sussex Fire and Rescue were called to the High Street shortly before 0230 BST on Saturday to find the second and third floors of the building alight.
Twenty firefighters were tackling the flames at their height, with crews called out from Worthing and Lancing.
The fire service said the blaze was being treated as deliberate.
Crews left the scene by about 0615 BST but were expected to return later on Saturday to inspect the state of the building.
Station officer Dave McMahon praised the actions of the firefighters who quickly extinguished the fire under "very hot and arduous conditions", stopping the flames spreading to neighbouring buildings.
The man taken to hospital was not thought to be in a serious condition.
Anyone with information about the fire is asked to contact Sussex Police.