Habitat factors were correlated with species richness for three guilds of lotic macroinvertebrates within the drainage basin of the Middle Fork of the Salmon River, Idaho. Species richness of filter feeders and scrapers increased with stream size whereas no significant longitudinal pattern was evident for gatherers. Annual variation in stream flow was inversely correlated with species richness for both filter feeders and scrapers, and food abundance was positively associated with scraper species richness. These relationships were consistent with the habitat templet concept of Southwood (1977). In contrast, our descriptive data gave no support for alternative, nonequilibrium models of community organization.