Anthropogenic changes frequently result in secondary contact and hybridization of species that have evolved in allopatry. The extent to which species hybridize, depends on the strength of premating isolating mechanisms: breeding behavior. Pupfishes are an ideal system to study the behavioral mechanisms that facilitate or restrict hybridization.  In the southwestern United States, the native Pecos pupfish (Cyprinodon pecosensis) readily hybridizes with an introduced congener, the sheepshead minnow (C. variegatus), in part because female Pecos pupfish prefer to mate with heterospecific males.  Secondary contact between Comanche pupfish (C. elegans) and sheepshead minnow results in a low incidence of hybridization, primarily because of assortative mating with conspecific males.  Comparative studies of mate choice patterns should provide insights into mechanisms that affect the divergence, maintenance, and introgression of lineages and allow predictions about susceptibility of species to hybridization upon secondary contact.