Reproductive interactions among species, such as attempts to mate with heterospecifics, can have profound impacts on patterns of geographic range and co-occurrence. For example, several theoretical models demonstrate that reproductive interference-the negative influence of heterospecifics on the abilities of conspecifics to successfully mate-can lead to parapatry, even across homogeneous landscapes, when the parapatric species are initially allopatric. However, the potential consequences of reproductive interactions on patterns of range limits and co-occurrence have been largely ignored by ecologists. Here, I use a combination of laboratory mating experiments and a genetic survey for two parapatric species of Skistodiaptomus copepods to evaluate the potential importance of interspecific reproductive interactions on the maintenance of parapatry. The genetic survey demonstrates phylogenetic exclusivity of these species, suggesting that gene flow has not occurred between them. Moreover, laboratory crosses between both species demonstrate that their parapatric boundary is not maintained as a hybrid zone because the two species cannot form viable hybrids. However, Skistodiaptomus oregonensis and S. pygmaeus males mate indiscriminately and interfere with the ability of heterospecific females to successfully reproduce. I suggest that reproductive interference results in priority effects that strongly influence the maintenance of this parapatric boundary and stress that reproductive traits are important and often-overlooked in ecological models for species range limits.