Stream physicochemistry and insect assemblages were studied in lowland tropical streams in Costa Rica to determine physicochemical variables explaining temporal and spatial variations in insect density and biomass. Streams drained a tropical wet forest landscape and had similar geomorphology, but differed in their water chemistry because of differential inputs of solute-rich (i.e., P, Cl^–, and Mg) geothermally modified groundwater. We hypothesized that, within a stream, physical factors, such as discharge, would result in temporal variation in insect assemblages, whereas interstream differences in solute composition would result in differences in insect assemblages among streams. Insect density, biomass, richness, and stream physicochemistry were measured monthly for 1 y in 6 streams. Insect samples were collected with a core sampler in runs where leaf litter overlying fine sediments was the main substrate. Streams with high inputs of geothermally modified groundwater had high conductivity and high concentrations of soluble reactive P (SRP). Annual temporal patterns in stream physicochemistry were related either to rainfall, with subsequent changes in discharge, or to the % geothermally modified groundwater entering streams. Streamwater pH decreased throughout the year, and unbuffered, nongeothermally modified streams changed from near neutral (pH >6) to near acidic (pH <4.5). In all streams, insect density and biomass were highest during the dry season and lowest during the wet season. Insect density and biomass were related positively to pH and to the number of days since the last storm. Insect taxonomic composition was similar among streams. Moreover, interstream variation in insect assemblages was small, and insect density and biomass were not related to any of the measured physicochemical variables, including SRP, which ranged from <5 to 267 μg/L. Overall, the physicochemical characteristics of the study streams were related mainly to geothermally modified groundwater inputs and seasonal patterns in rainfall. Streams with geothermally modified groundwater inputs were better buffered against pH changes than the other streams. Insect assemblages reflected fluctuations in stream physicochemistry during the year and were associated with 2 main factors: floods and pH. Physicochemical characteristics were very different among streams, but insect assemblages were not related to them. We hypothesize that the heterogeneous nature of benthic substrata in the streams resulted in high variation in density and biomass of insects, potentially obscuring differences in insect assemblages among streams.