Firefighters were called to Wendron yesterday afternoon after a report of 30 hay bales on fire.
One appliance and the water carrier from Helston were sent just before 4pm after a report that the hay bales were well alight.
Crews used water jets to extinguish the fire.
A fire service spokesman said: "At this time the cause is believed to be arson and Devon and Cornwall Police have been notified."
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