The endangered Pecos gambusia (Gambusia nobilis) is known to aggregate around spawning pairs of the endangered Leon Springs pupfish (Cyprinodon bovinus). We documented the behavior of both species, and the reproductive success of C. bovinus, following exposure to controlled numbers of G. nobilis. We found higher densities of G. nobilis around spawning pupfish compared to nonspawning pairs yet there was an absence of a negative density-dependent effect on C. bovinus reproductive success. Gambusia nobilis density showed no effect on spawning behavior yet interference by G. nobilis might reduce the time male C. bovinus spend pursuing females and eliciting spawning behavior.