The fire overnight destroyed a garage and two cars inside.
Fire Chief Gary Buchberger said Monday there are a lot of similarities to the earlier arson fires.
Police believe a serial arsonist has set 12 fires, mostly to detached garages, on the city's east side since May 28. One garage fire was on the west side.
The arson fires are costing the city's police department in overtime pay. Wausau police investigators earned $19,000 in overtime pay in June and July, compared with 4,000 in the same period last year.