We present baseline data on the distribution and abundance of invasive red-eared slider turtles (Trachemys scripta elegans) in the Sacramento River basin of northern California and discuss their possible impacts on the native western pond turtle, Emys [ =  Actinemys] marmorata. Although T. s. elegans may be able to spread throughout the drainage, our surveys suggest that it is currently concentrated near urban areas and relatively rare throughout most of the areas where large Emys marmorata populations persist. We also confirm 2 additional localities where extralimital breeding of T. s. elegans is occurring.