The emergency services are attending the scene of five fires in fields close to the new A43 bypass close to Little Stanion.
Crews from Corby fire station were called out at 1.32pm to the field between the Eurohub roundabout and Stanion where they discovered five different hay bales had been set alight.
Police also attended at 2.10pm after smoke drifted across the A43. Although the road is still open, motorists are being warned to proceed with caution.
Officers said they were treating the fire as suspicious and deliberate.
Anyone with information can call police on 111 or Crimestoppers on 0800 555 111.