Many genes involved in reproduction have evolved at a rapid rate in diverse taxonomic groups. We use new data for the mammalian reproductive gene that encodes acrosin to document patterns of adaptive evolution and rapid divergence. Likelihood scores for neutral and selection models indicate that molecular evolution of the Acr gene is driven by positive selection. Comparisons of Acr sequences to mitochondrial genes and a nuclear intron show that reproductive genes can be informative phylogenetic markers. In bovid artiodactyls (cattle, sheep, goats, and antelopes), exon 5 of Acr is highly variable, is characterized by a low level of homoplasy, and has a fairly even spread of substitutions across the 3 codon positions. Impressive congruence with independent DNA data sets suggests that Acr, and perhaps additional rapidly evolving reproductive molecules, show promise for resolving evolutionary radiations of mammals.