A 13-tonne excavator and a JCB digger have been destroyed in two separate blazes on the same evening.
The fires happened within hours of each other in Eastway, Fulwood and Sandy Gate Lane, Broughton.
Suspected arsonists struck at a worksite off Eastway, close to the Asda store, at 6.30pm yesterday (Thursday, April 11).
Firefighters from Preston and Penwortham were called to the worksite after a 13-tonne excavator was set alight.
Fire investigators said the cause of the blaze is thought to be deliberate. It happened shortly after work crews had left the site for the day.
Excavators of this size can cost as much as £50,000.
A spokesman for Lancashire Fire and Rescue Service said: "Fire crews from Preston and Penwortham attended reports of a mechanical digger on fire.
"When they arrived they quickly dealt with the fire which involved a thirteen tonne excavator using two breathing apparatus and one hose reel.
"Firefighters were in attendance approximately 55 minutes.
"The cause of the fire is thought to have been started deliberately and an investigation is underway."
Just four hours later - at 10.50pm - the Preston fire crew were called out again to reports of an JCB digger on fire in a field in Broughton.
Firefighters found that a large bonfire had been lit close to a JCB digger, around 700m from the road in Sandy Gate Lane.
It is believed that radiated heat or a rogue spark from the bonfire had spread to the diesel-powered digger, fuelling an even bigger blaze.
A fire crew from Penwortham was called in to assist and the two units battled the blaze, near Broughton High School, for around two hours.
A spokesman for Lancashire Fire and Rescue said: "Fire crews from Preston and Fulwood attended a fire involving a digger on some open land near Sandy Gate Lane, Broughton.
"Upon arrival, firefighters located the digger on fire approximately 700m from the roadway, along with a large bonfire.
"They used one hose reel to extinguish the fire and were in attendance for approximately two hours.
"An investigation is now underway and police have been informed."
The fire service said that the first fire in Eastway appeared to have been caused deliberately, whilst an investigation into the Broughton fire is ongoing.
Both fires are situated on sites earmarked for residential and commercial development.
A spokesman for Lancashire Police said: "There is no suggestion of any further link between the two incidents."
Arsonists have struck at a number of sites in Preston this week.
On Wednesday (April 10), two former golf clubs were targeted by arsonists. The clubhouse at Fishwick Hall Golf Club was set alight at 2pm, followed by a fire at Ingol Village Golf Club later that evening.
Both incidents are being investigated by police, but are not believed to be related.