Arsonists set fire to hundreds of tonnes of straw in Longstanton.
A crew from Cottenham was called at about 9.20pm on Tuesday to a field off the B1050 amid reports of a straw stack fire.
A spokesman for Cambridgeshire Fire and Rescue Service said: "Crews arrived to find around 300 tonnes of straw well alight.
"Firefighters used one hose reel, beaters and drags to prevent further spread and contain the fire. The crews will be regularly re-inspecting the incident."
An investigation revealed the cause of the fire to be deliberate. Anyone with information should contact police by visiting www.cambs.police.uk to submit an online report or call Crimestoppers on 0800 555111.