Background/Question/Methods The Cape Horn Biosphere Reserve (CHBR) in southern Chile is a remote area that is one of the planet's last remaining wilderness areas and experiences almost no atmospheric contamination. Consequently, little previous work has been done to study the ecology and biodiversity of these “pristine” streams. Since 2003, scientists at the Omora Ethnobotanical Park (OEP) have been sampling aquatic macroinvertebrate fauna in the Róbalo River to determine basic questions, such as biogeographic affinities and community structure, as well as ecological processes and patterns, such life history traits and species interactions. This watershed, which provides drinking water to the world's southernmost town Puerto Williams, serves as the scientific and education center of the CHBR and is an ideal site to conduct long-term ecological studies as it hosts a representative mosaic of Cape Horn's ecosystems and constitutes a continuous altitudinal gradient from mountain peaks to shore within the OEP. Previous data show that aquatic invertebrates inhabiting these streams exhibit a mix of biogeographic origins (Gondwanic, Neotropical, Andean) and a high level of endemism at the genus and species level. To study the effect of elevation and vegetation type on aquatic diversity, we sampled macroinvertebrates at six altitudinal levels (from headwaters to river mouth) from December 2007 to January 2008 (austral summer). At each station, we deployed an array of sampling techniques (malaise traps, Surber and D-frame samplers) to provide a comprehensive characterization of the macroinvertebrate assemblage and life stages in the habitat type and elevation being sampled. We analyzed taxonomic richness at each altitudinal level to characterize zoning patterns and compared the macroinvertebrate distribution patterns with vegetation communities and physical and chemical parameters. Results/Conclusions The benthic macroinvertebrate community included mostly insect taxa belonging to the orders Ephemeroptera, Plecoptera, Trichoptera, and Diptera (Chironomidae), along with other invertebrates groups such as Amphipoda, Bivalvia and Gastropoda. Representatives of these taxa were present at all altitudinal levels, but relative abundance and species identity changed along the stream gradient. Also, a delay was observed in the development of immature stages of Ephemeroptera, Plecoptera, and Trichoptera collected at higher altitudes. While sharp changes were not observed in community structure along the altitudinal gradient, differences in life histories, and potentially ecological interactions, could be an important effect of changes along the stream length from headwaters to the river mouth. These data will be complemented by a winter survey (May-August 2008) to further characterize taxonomic, phenological and ecological patterns.