Sperm competition is widespread, resulting in sexual selection when the gametes of two or more males compete for access to a given set of ova. This study shows, for the first time, that sexual selection has influenced the evolution of the mammalian vas deferens – that is, the muscular duct which rapidly transports spermatozoa from the cauda epididymis to the ampulla during sexual activity. Morphometric studies of 103 species, representing 70 genera of mammals, show that the vas deferens is on average 33% shorter and its muscular walls are 45% thicker, in those forms where sperm competition pressures are greatest. The outer and inner longitudinal muscle layers of the vas are thickened under these conditions, whereas the central, circular muscle layer is significantly reduced. Thus, mammals that have multiple partner mating systems, and large testes in relation to body weight, have shorter and more muscular vasa deferentia than mammals where single partner mating systems, and smaller relative testes sizes, are the norm. These differences are statistically significant after application of procedures to correct for possible phylogenetic biases in the data set. Sperm competition has played an important role in the evolution of the mammalian vasa deferentia, favouring muscular specializations that enhance sperm transport.