Zibethacarus beeri sp. nov., is described and illustrated from adults collected from the nest of Microtus oregoni (Bachman) (Rodentia) in western Oregon, U.S.A. Males are approximately half the size of females, and samples collected into alcohol using a Berlese funnel contained males still clinging to the dorsum of females, their modified tibiae of legs II clasping the female's modified dorsal setae h₂. It is probable that such persistent clinging behavior is a result of mate guarding on the part of the male.