Damage is estimated at $3-million in Tuesday's fire at Huybers Hog Haven Ltd. in Plympton-Wyoming.
The blaze at the commercial operation in the 5600-block of Confederation Line broke out just before 9am.
There were no injuries to humans reported, but about 4,000 pigs perished in the blaze that destroyed a 100-by-400 ft barn.
Lambton OPP Constable Travis Parsons says firefighters from Brooke-Alvinston, Warwick and Plympton-Wyoming battled the blaze for several hours.
"Plympton-Wyoming Fire Department have been on scene all night, they're still trying to extinguish some hot spots in the barn," says Parsons.
"As of now, nothing suspicious has been revealed and the investigation is still ongoing."
The Ontario Fire Marshal's Office has been contacted.