Around 100 straw bales were destroyed and farm equipment damaged after a barn was deliberately set on fire.
The building was set alight at Setonhill Farm in Longniddry, East Lothian, around 5pm on Tuesday.
No one was injured, but a tractor and other agricultural equipment were damaged.
Police believe the blaze was started deliberately and have appealed for information.
Detective sergeant Declan Hogg said: “This fire is believed to have been started deliberately and we are carrying out enquiries to establish the full circumstances.
“We would ask anyone who may have seen any suspicious activity around the area between 4pm to 6.30pm on Tuesday afternoon or who may have dashcam footage of anyone walking on the roads in the Longniddry area around that time to let us know.
“Any information can be passed to police by calling 101, quoting incident 2528 of July 28, or call Crimestoppers on 0800 555 111.”