Biotic invasions can have strong direct effects on native biota, which can in turn have profound implications for native populations and communities.  Brook trout are among the most broadly introduced fishes, invading small native cutthroat trout streams throughout the western U.S.  Brook trout mature earlier than cutthroat trout, and their fry emerge earlier, giving them a longer growing season.  Therefore, we hypothesized that streams with nonnative brook trout would have greater fish density, biomass, and production than streams with cutthroat trout.  We addressed these hypotheses by comparing ten pairs of streams within two regions in the northern and central Rocky Mountains. Streams were paired based on physical characteristics and the presence of brook trout or cutthroat trout in allopatry.  In accordance with our hypotheses, streams with brook trout had consistently higher fish density, biomass, and estimated production than those with cutthroat trout, regardless of region.  Brook trout density in the central Rocky Mountain streams was significantly greater than brook trout density in the northern streams.  We are currently conducting research to address consequences of brook trout invasion, such as effects on benthic invertebrates, which could have cascading effects on other components of aquatic or riparian food webs.