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Extra-group paternity, in which offspring are sired by a male outside the breeding group, may alter the distribution of reproductive success in a population, thus affecting the opportunity for sexual selection. Both inter- and intraspecific studies have focused largely on mating systems in which females choose their social mates, and less is known about extra-group paternity in polygynous systems in which social mates are largely dictated by male-male competition. In this study, we examine the frequency and distribution of extra-group paternity in a harem-forming bat, Phyllostomus hastatus. We find that despite aggressive harem defense, males are unable to fully monopolize reproduction within their harem and over 70% of harems contain extra-group offspring. Harem males in better body condition suffered less paternity loss, but we found no effect of male age or body size. Even though the …