regulation of rivers and streams modifies parameters that directly influence on the biota. in achiras-del gato endorheic basin has completed the construction of a midsize dam that will alter the limnological characteristics of the system and the structure of benthic communities. this study provides the first taxonomic records of macrobenthos of this lotic system and analyzes the spatial and temporal variations of the community structure in the foothills stretches of achiras stream. zoobenthos samples were collected and abiotic variables were recorded in runs and riffles, during a year, in two sites located upstream and downstream of the dam area, prior to the dam was built. for each site, habitat and season we determined the abundance, frequency, taxonomic richness, shannon and evenness, and the data were analyzed using one-way and three-way anova. attributes of the reference communities were complemented by the determination of ecological indicators and detectors, and exclusive taxa. in the first site 73 taxa were recorded, simulium was the most abundant and leptohyphes eximius eaton, 1882, was the most frequent. in the second site we found 75 taxa, caenis was the most abundant and cricotopus sp. 3, was the most frequent. in this study composition and distribution of the biota and indicators taxa are discussed. we determined that the community attributes were affected by the hydrological periods in wich the samples were taken. achiras stream has a phreatic-pluvial hydrological regime with low flows in winter and high flows in summer. therefore, we expect changes in native biota and in system integrity in response to hydraulic variability reduction when achiras dam start functioning and alters the natural flow.