Quantifying interactions between nonnative and native species can aid the conservation of biodiversity.  The Gila River system provides a unique opportunity for examining these interactions because many anthropogenic alterations (e.g. dams, pollution) are absent.  The objective of this study was to assess how biomass of nonnative fish species and algal biomass influence the total biomass of the native fish community across 6 sites during spring, summer, and autumn of 2008.  We predicted that biomass of native fishes would increase with algal resource availability, unless increased algal biomass also supported higher biomass of nonnative predators.  Algal biomass was most variable across sites during spring (5.2-51.8µg/cm² chl a) with a general downstream increase, but became more homogenous across sites during summer and autumn (5.7-13.4µg/cm² and 5.1-20.2µg/cm² respectively).  Nonnative species were present at all sites during all seasons, with mean non-native biomass being consistently highest at the most headwater site (1.7g/m²) and consistently lowest at a downstream site (0.24g/m²).  Contrary to our predictions, mean native fish biomass (4.8g/m²) was greatest where mean algal biomass was lowest (6.1µg/cm²) and mean nonnative biomass was greatest (1.7g/m²).  Results suggest factors other than basal resource availability and nonnative biomass may limit the occurrence of native fishes.