We examined seasonal and diel variation in prey species composition and biomass in the diet of the exotic rainbow trout Oncorhynchus mykiss (Walbaum, 1972). The study was carried out in the upper-forested sections of a low order stream in Andean-Patagonia. We studied the importance of functional feeding groups of aquatic invertebrates and the relative contribution of terrestrial and aquatic prey items in order to assess the pathways connecting terrestrial and aquatic ecosystems along a stream food web. Trout fed on approximately 40 invertebrate species and scrapers were consistently selected, suggesting their increased vulnerability to predation. However in terms of biomass, rainbow trout diet was mostly composed by shredders which emphasized the role of the allochthonous plant detritus pathway in food webs of forested small streams. Trout individuals fed more intensively in spring and summer and during daytime. Terrestrial items constituted a minor proportion of the diet implying that this component did not represent a significant subsidy for this fish population. As a consequence, the top-down effect on the aquatic community does not appear to be dampened since trout do not strongly preyed on terrestrial invertebrates.