We sampled 8 sites (riffles) in a parent stream and its major upstream tributary in northern Michigan to quantify environmental heterogeneity or gradients to identify spatial determinants of 5 species of larval hydropsychid caddisflies. Abundance patterns of species varied at both small (microhabitats) and intermediate (sites) spatial scales, as did key determinants of abundance. Substrate size and type were important in affecting species microdistributions. A selectivity index (L) showed that Hydropsyche betteni and Ceratopsyche sparna, the 2 most abundant species, were disproportionately more abundant on boulders and logs, respectively, than occurrence of these substrates at sites. Cheumatopsyche spp. also exhibited a propensity to use logs but avoided boulders. Microdistributions of Ceratopsyche inorosa and C. slossonac were more variable than these other species. Nonetheless, all 5 species occupied gravels in disproportionately lower frequencies than occurrence of this substrate at sites. Current velocity explained a small but significant amount of variation (4-18%) of densities on microhabitats for all species but C. morosa. The presence of the macroalga Cladphora glomerata significantly increased larval densities on substrates but had no significant effect on overall site densities. Stepwise regression analyses revealed that amount of habitable substrate, seston quantity, and temperature were key predictors of site assemblages along the upstream-downstream gradient. The finding that amount of habitable substrate was significantly related to site densities of all species, except C. morosa, emphasizes the large impact suitable habitat has on larval hydropsychid densities in these sandy streams. Site densities of C. morosa were negatively associated with seston quantity, whereas those of C. sparna were positively associated with seston amount and kind. Ceratopsyche sparna were predominant in downstream reaches, where detrital (leaf) seston loads were relatively high. Site densities of H. betteni and C. slossonae were positively correlated with algal seston and temperature. These 2 factors were affected by proximity to lake outlets and beaver dams. Our study shows that multiple factors operate synergistically over several spatial scales to influence hydropsychid species distributions.