Inbreeding effects on fitness have most often been quantified via juvenile mortality. However, inbred adults may suffer from inbreeding depression if their fertility or fecundity is compromised as a consequence of inbreeding. Here, the effects of inbreeding on male fertility in oldfield mice, Peromyscus polionotus, were examined. Testicular sperm concentration was assessed in 93 males, 68 of which were paired for breeding. Forty of the 68 paired males failed to produce offspring. Total testicular sperm count, sperm count (g testis)(-1), and testis mass all declined significantly with increasing inbreeding coefficient. Sperm concentration did not significantly impact reproductive success. Although sperm concentration in males of most species can decline to low levels before reproductive impairment is detectable, the declines in testicular sperm concentration found here suggest that inbreeding can affect fertility in adult males. Furthermore, monitoring testicular sperm concentration could provide a mechanism to monitor potential declines in reproductive performance before population-level reproductive success is irreparably impaired. The implications for the management of small, captive and wild populations may be substantial.