During 2010–2018 we studied the relative abundance of invasive red–eared sliders (Trachemys scripta elegans) and native western pond turtles (Emys marmorata) at Clear Lake and its associated wetlands in Lake County, California, where T. s. elegans was first detected in 1968. Both species coexisted throughout the lake and its associated wetlands, but the proportion of T. s. elegans was significantly higher in the eastern arm of the lake (59% vs. 41%, n = 135) than in the northwestern arm (40% vs. 60%, n = 181) and, especially, the southeastern arm (10% vs. 90%, n = 31). Geographic differences in the proportions of the species were unrelated to the degree of urbanization and are presumably due to differences in the timing or frequency of T. s. elegans introductions in each arm of the lake.