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Milkweed leaf beetles (MLBs; Labidomera clivicollis Chrysomelidae) are parasitized by a subelytral mite, Chrysomelobia labidomerae (Tarsonemina: Podapolipidae). We show that C. labidomerae is transmitted between MLBs when they copulate, and can reduce the survival of nutritionally stressed beetles. We investigated the effect of this sexually transmitted parasite on mate choice and male-male competition in MLBs, and the consequences of variation in these behaviours for mite transmission. We found no evidence of parasite avoidance by MLBs, and evidence for high rates of parasitism in the MLB populations we surveyed. In the absence of females, parasitized males contacted unparasitized males more often and for longer than controls, and they tended to displace rival males from females more often than did unparasitized males, a result consistent with the interpretation that parasitized beetles compensate for loss of fitness by increasing reproductive effort. These changes can also benefit mites, because longer and possibly more contacts between beetles provide more opportunities for transmission, but there is no evidence that these changes in male behaviour result from parasite manipulation.