Officials say two buildings in Ronkonkoma were gutted by flames about three hours apart from each other.
Officials say a vacant building on Portion Road was engulfed around 3 a.m.
At least five fire departments have been battling the blaze. No serious injuries have been reported.
Meanwhile, more than half-dozen fire departments battled a house fire on Waltess Road just before 11:30 p.m. last night. There are no reports of injuries.
It is not clear what started the fires.