We estimated population abundance and sex ratio for a freshwater live-bearing fish species (Poeciliopsis baenschi) that inhabits fluvial systems in western Mexico. We conducted mark-recapture experiments in 6 distinct populations. We used both our catch data as well as estimates of capture probability to calculate the number of individuals per unit water volume for adult males, adult females, and juveniles. Overall, abundance varied among sites from 23.8 individuals · m^-3 to 363.2 individuals · m^-3. In most of our study sites, the estimated sex ratio was markedly biased toward females (up to 11 females per male). We discuss the potential causes and implications of the observed demographic features.