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Research on the role of parasites in sexual selection has focused mainly on host mate choice favoring relatively unparasitized males. But parasites can also generate variance in host reproductive success by influencing the ability of individual hosts to directly compete among themselves for mates or fertilizations, a subject area that has received far less attention. We demonstrate experimentally that parasitism by mites can drive sexual selection by way of a novel mechanism involving male competition: physical inhibition of host copulation. Mite resistance in natural populations is heritable, emphasizing the evolutionary potential of parasite-mediated sexual selection in this system and indicating that females should be receiving indirect fitness benefits as a result of this process. We show that parasitism by mites, Macrocheles subbadius, reduces mating success of male Drosophila nigrospiracula. Smaller males were more strongly compromised, identifying host body size as a tolerance trait. As parasite load increased, the rate at which males attempted to copulate but failed because of obstruction by mites increased. When mites were removed from infested males, host mating success was restored. Thus, the physical presence of the mites per se generates differential mating success, in this case by interrupting the normal flow of mating behaviors. This study elucidates a potent mechanism of parasite-mediated sexual selection in a system wherein parasite resistance is demonstrably heritable, and as such expands our understanding of the evolutionary potential of sexual selection. [ABSTRACT FROM AUTHOR]