Abstract: When close relatives are forced to reproduce, the resulting offspring inherit above average homozygosity and reduced fitness [1Falconer D.S. Mackay T.F.C. Introduction to Quantitative Genetics.4th Edition. Longman, New York1996Google Scholar, 2Charlesworth B. Charlesworth D. The genetic basis of inbreeding depression.Genet. Res. 1999; 74: 329-340Crossref PubMed Scopus (550) Google Scholar]. Biologists now recognize inbreeding depression in the wild [3Crnokrak P. Roff D.A. Inbreeding depression in the wild.Heredity. 1999; 83: 260-270Crossref PubMed Scopus (653) Google Scholar, 4Keller L.F. Waller D.M. Inbreeding effects in wild populations.Trends Ecol. Evol. 2002; 17: 230-241Abstract Full Text Full Text PDF Scopus (2271) Google Scholar, 5Saccheri I. Kuussaari M. Kankare M. Vikman P. Fortelius W. Hanski I. Inbreeding and extinction in a butterfly metapopulation.Nature. 1998; 392: 491-494Crossref Scopus (1266) Google Scholar], a phenomenon that will probably increase as natural populations become depleted and fragmented. Inbreeding depression is most commonly expressed as compromised fertility and embryogenesis [4Keller L.F. Waller D.M. Inbreeding effects in wild populations.Trends Ecol. Evol. 2002; 17: 230-241Abstract Full Text Full Text PDF Scopus (2271) Google Scholar], but actual mechanisms remain poorly understood, especially for wild populations. Here, we examine how reduced heterozygosity influences spermatozoal and gonadal traits in wild rabbits (Oryctolagus cuniculus) sampled across the United Kingdom. By using a suite of 29 microsatellite markers (analyzed to confirm representation of individual heterozygosity across our sample), we found a significant negative relationship between heterozygosity and the production of normal sperm; the relationship was significant both between (n = 12) and within (n = 91 [total males], 42 [island], 49 [mainland]) populations. Reduced heterozygosity was also associated with decreased testis size across males (n = 112), but no relationship was seen at the population level, suggesting environmental confounds. Our results show, for a wild mammal, that inbreeding is associated with decreased sperm quality, confirming suggestions of links between inbreeding and elevated sperm abnormalities in rare felids [6Barone M.A. Roelke M.E. Howard J. Brown J.L. Anderson A.E. Wildt D.E. Reproductive characteristics of male Florida panthers: comparative studies from Florida, Texas, Colorado, Latin America, and North American zoos.J. Mammal. 1994; 75: 150-162Crossref Scopus (106) Google Scholar, 7Wildt D.E. Bush M. Goodrowe K.L. Packer C. Pusey A.E. Brown J.L. Joslin P. O'Brien S.J. Reproductive and genetic consequences of founding isolated lion populations.Nature. 1987; 329: 328-331Crossref Scopus (253) Google Scholar, 8Wildt D.E. Bush M. Howard G. O'Brien S.J. Meltzer D. VanDyk A. Ebedes H. Brand D.J. Unique seminal quality in the South-African cheetah and a comparative evaluation in the domestic cat.Biol. Reprod. 1983; 29: 1019-1025Crossref PubMed Scopus (195) Google Scholar]. These findings could explain why inbreeding depression so frequently arises via compromised fertility and embryogenesis [4Keller L.F. Waller D.M. Inbreeding effects in wild populations.Trends Ecol. Evol. 2002; 17: 230-241Abstract Full Text Full Text PDF Scopus (2271) Google Scholar].