Firefighters have put out a blaze at the site of Scarborough's former Military Adventure Park.
Two engines were called at around 3pm to the derelict site at Peasholm Gap on Saturday.
Firefighters used cutters to gain access to the premises so they could tackle the flames.
A spokesman for North Yorkshire Fire and Rescue added: "Scarborough crews attended and dealt with a fire in a derelict building. Two hose reel jets used to extinguish.
"Cause believed deliberate by youths, incident left with Police."