Although the importance of water temperature to the ecology of stream fishes is well documented, relatively little information is available on the extent of regional variation i nthermal regime and its influence on stream fish distribution and abundance patterns. In streams draining the heterogeneous glacial landscape of Michigan’s Lower Peninsula, regional variation in summer mean temperature and temperature fluctuation is among the highest reported in the literature. We developed a habitat classification to simplify the description of thermal regimes and to describe the relationships between available thermal regimes and distribution patterns of stream fishes. Changes in community composition, species richness, and standing stocks of key fish species occurred across gradients in mean temperature and temperature fluctuation. These changes were used to identify three mean temperature categories (cold, < 19 C; cool, 19-<22 C; and warm, >= 22C) and three temperature fluctuation categories (stable, <5 C; moderate, 5-<10C; and extreme, >=10C). The combination of these categories resulted in a 3 x 3 matrix with nine discrete thermal regimes. The classification developed in this study provides a framework for descriptions of the realized thermal niche of stream fishes, and can be used as a baseline for measurement of changes in distribution patterns associated with future climate warming. Our results suggest that observed differences in community structure among sites are largely attributable to spatial variation in mean temperature and temperature fluctuation. Thus, accounting for the linkage between regional variation in thermal regime and fish community structure should imporove our ability to effeectively assess and manage stream resources.