The strength of biotic interactions between native and invasive species is a key component of invasive species impact, but often is not quantified explicitly. We measured biotic interaction strengths between an invasive and a native endemic snail species in 2 streams in the western United States. The invasive freshwater New Zealand mudsnail, Potamopyrgus antipodarum, was the dominant snail in a 3^rd-order stream, and a closely related endemic snail, Pyrgulopsis robusta, was the dominant snail in a 1^st-order tributary stream. We compared consumer–resource (C–R) and consumer–consumer (C–C) per biomass interaction strengths in both streams using field enclosure experiments. Consumer effects on algal resources were strong and equivalent for both snail species in both streams. Invasion success of P. antipodarum could not be attributed to an unusually high rate of resource acquisition at either site because the 2 snails exerted equivalent C–R interaction strengths. However, C–C interactions were asymmetric; the invasive snail significantly limited growth of the native snail, whereas the native snail facilitated growth of the invasive snail. These effects were stronger in the stream where P. antipodarum was dominant. Thus, per biomass interaction strengths indicated that asymmetric interactions could be contributing to patterns of dominance by invasive and endemic stream snails.