The dimensions of testes, epididymides and spermatozoa of Australian dingoes (Canis familiaris dingo) and a sample of domestic dogs (Canis familiaris familiaris), chosen to lie within the range of dingo body weights, were compared. There were no significant differences for body weight or testis weight but total epididymal weight, the caput + corpus weights and sperm numbers in the cauda epididymidis were significantly larger in dogs. Dimensions of the spermatozoa were similar for dogs and dingoes except for a small but significantly larger head length in dingoes. Seminiferous tubule diameters were significantly larger in dingoes and epididymal tubule diameters were similar in dogs and dingoes except for significantly wider tubules in the cauda of dogs. The estimated length of tubules in the total epididymis and in the cauda was significantly greater in dogs than in dingoes. Greater size of the epididymis and especially of the cauda epididymidis probably reflect selection for larger sperm stores in free-ranging domestic dogs where there is a greater chance of intermale sperm competition. Two possible hybrids (determined by skull morphometry) were intermediate between dogs and dingoes for some of these dimensions.