Toxicum funginum Lewis, 1894 (Coleoptera: Tenebrionidae) is a medium-sized, horned species that lives in Japan and Taiwan and feeds on bracket fungi. Intraspecific behavioral interactions among adults were examined in the laboratory. Males engaged in combat with other males using paired horns located between their compound eyes. The contact surfaces of the horns bore curled brown hair, possibly to absorb physical impact. Larger males with longer horns won more frequently than smaller individuals with shorter horns. Smaller males rarely engaged in combat. Females pushed away approaching males violently with their heads.