Fire crews and volunteers are tackling 12 separate blazes across seven miles of Dartmoor in Devon.
The worst of the fires has flames stretching along a three-mile front.
Devon and Somerset Fire and Rescue Service was originally called to an area south of Princetown on Monday evening.
Crews started battling the fires again at 06:20 BST with help from National Park rangers and local farmers. A police helicopter is also assisting.
The fires have broken out between Green Hill and Nakers Hill and around Nun's Cross Farm and Great Nats Head.
Crews are currently using beaters across several fire fronts.
Alan Gilson, from Devon and Somerset Fire and Rescue Service, said conditions were "pretty good" for the teams.
"We've got about eight crews working on the moor," he said.
"There are several seats of fire and the one giving us the biggest problem at the moment is about a two-to-three-mile flame front."
Specialist moorland fire fighting equipment and 4x4 vehicles have been deployed to various locations within the area to help halt the flames, the fire service added.
The National Police Air Service (NPAS) is assessing the spread of the fire from above to help crews on the ground prioritize their work.