I investigated interactions between native and nonnative fishes in the upper Muddy River system to add insight into (1) the mechanism causing the decline of the Moapa daceMoapa coriaceaafter the introduction of the shortfin mollyPoecilia mexicana, (2) the reason Moapa White River springfishCrenichthys bailevi moapaewere less affected by the introduction, and (3) the reason interactions between natives is relatively benign. I investigated the hypothesis that the shortfin molly caused the decline of the Moapa dace through competition or predation on larvae, pressures not experienced by the Moapa White River springfish. Relative interspecific competition was analyzed by contrasting the ranges of spatial and dietary overlap among larval, juvenile, and adult life stages. There appeared to be moderate to low spatial overlap between the various life stages of native and nonnative fishes. Overlap in diet was highest between adult Moapa White River springfish and shortfin mollies. Laboratory experiments suggested that shortfin mollies prey vigorously upon fish larvae. In terms of spatial habitat use, Moapa White River springfish larvae were less available to adult shortfin mollies for consumption than were Moapa dace larvae. When predation on larvae is the mechanism by which nonnative fish reduce native forms, aggressiveness of the predator and the degree to which the predator overlaps in habitat with the prey may influence the degree to which a native fish population is affected.