The species-area pattern for plants and animals predicts larger areas generally contain greater species richness. Also, it is recognized that the number of species increases from north to south with temperature. Our analyses of the distribution of Michigan fishes support these observations. The common log-log slope value of about 0.25 for species-area relationship was found for the Upper and Lower peninsulas of Michigan as well as the Great Lakes watersheds of Superior, Huron and Erie (Michigan was an exception with a value of 0.44). The number of species per 2500 square kilometers increased from 50 species for the Upper Peninsula to 73 species for the southern Lower Peninsula as the latitude decreased from 47° to 42°, and the Great Lakes watersheds showed a similar pattern. The increase in species number from north to south correlated with a measured increase in growing degree days. The multiple regression of area and growing degree days of watersheds against number of species resulted in an increase in adjusted r² to 0.67 from 0.38 for the regression of species-area alone. Species diversity of Michigan fishes is strongly influenced by watershed area and growing degree days temperatures.