The Pecos gambusia (Gambusia nobilis) is an endangered poeciliid native to western Texas and eastern New Mexico. The decline of G. nobilis in Texas is likely the result of habitat alteration and introduction of potentially invasive congeners (e.g., the introduced largespring gambusia, Gambusia geiseri). In the East Sandia Cienega, observers have noted mating between G. geiseri and G. nobilis. Because potential hybridization between native and introduced species could have detrimental effects on reproductive fitness, we compare mean brood size, gonadosomatic indices, and standard length of both G. geiseri and G. nobilis as a first step in assessing the impacts of G. geiseri introduction in this area.