OFFICIALS said the pool at Ashbourne Leisure Centre has been closed following a fire there last night.
Firefighters said the Clifton Road building was left severely smoke-damaged after the fire - which had been started deliberately in a bin - had spread to the building.
A man was later arrested on suspicion of arson.
The fire started on Saturday night just before 11pm.
A spokesman for Derbyshire Dales District Council said: "A fire at our Ashbourne Leisure Centre late on Saturday night means the pool is closed while a damage investigation takes place.
"Our dryside facilities, including fitness suite, are open as usual.
"The full extent of the damage won't be assessed until Monday, but the plant room was affected, including the pool filtration system.
"At one stage the blaze - thought to have started outside the building - spread to the roof but, fortunately, the centre's new changing village, currently nearing completion, does not seem to have been affected.
"An arrest has been made and police are dealing with the incident."