We explored the effects of introduced trout on primary and secondary producers at 2 spatial scales in 17 Andean stream reaches above 3000 m asl in 3 subbasins of the Guayllabamba watershed in Ecuador. At each stream reach, we measured trout density, biological factors, and environmental factors that might affect distribution and density of trout. We also carried out a field-inclusion experiment along a 105-m section of the Saltana stream (which has no trout), in which we introduced trout in experimental cages to assess their effects on aquatic invertebrate populations and algal biomass. Oncorhynchus mykiss (Rainbow Trout) is widely distributed in streams of the Andean páramo in northeastern Ecuador and was the only fish species found. Trout distribution was correlated with altitude, quality of riparian vegetation, substrate diversity, and conductivity. In the survey, invertebrate community composition did not differ between streams with and without trout, but trout density and invertebrate richness were positively related. Trout density and algal biomass were negatively related. In the experiment, trout reduced the density of invertebrates (predominantly Andesiops [Baetidae], Orthocladiinae, and Simuliidae sp. 1) in drift and benthic samples. Thus, trout appeared to have a significant effect on aquatic invertebrate communities at the local scale via direct predation, which affects the densities of certain taxa, and behavioral responses of certain invertebrates (i.e., reduced drift). However, trout did not appear to affect algal biomass. This result might have been a consequence of the limited time that trout were kept in the enclosures. We conclude that introduced Rainbow Trout in tropical high-altitude streams can strongly affect invertebrate behavior and densities, which in turn, might affect other levels of the food web (as seen in the survey), but their effect on primary producers remains to be tested.