In the region of Cheboygan County, Michigan, the forked fungus beetle, Bolitotherus cornutus (Panzer), had one generation per year. The beetle overwintered both as adults and larvae; the adults under bark, the larvae in the fungus host, chiefly Fomes applanatus. Overwintering adults laid eggs in mid June, the resulting larvae completing their development in early fall. The overwintering larvae became adults in mid sumer and began ovipositing in late August. The larvae from these eggs developed until weather condtions retarded them. The eggs were deposited singly on the upper and lower surfaces of the host fungus and were covered with an excrement-like capsule. Each female deposited between 8 and 12 eggs at a rate of 1 or 2 eggs per day. The larvae bored into the host fungus and tunneled through it during development. Pupation occurred in an enlarged chamber in the larval tunnel. The larval reared in the laboratory on pulverized Fomes applanatus underwent four instars befroe pupation. Measurements of head capsules of field collected larvae indicated that five instars may occur in nature. The forked fungus beetle is nocturnal, feeding and mating chiefly at night. A peculiar noise making behavior occurred during the mating season. A braconid wasp, Eubadizon orchesiae (Ashm.) was found in the pupal stage in old larval tunnels, with the head capsule of a third instar beetle larva at one end of its cocoon. B. cornutus larvae were also found dead in the center of a white mycelial mass in the larval tunnels.