Incidence of black spot disease was measured qualitatively in fish collections from 10 stream systems around Toronto, Ontario. Individuals in 18 of 49 fish species were found with black spot infections; they included representatives of Salmonidae, Catostomidae, Cyprinidae, Centrarchidae, and Percidae. Fishes in the families Petromyzontidae, Umbridae, Ictaluridae, Gasterosteidae, and Cottidae were all collected in substantial numbers but were never observed with black spot. Measures of stream size, gradient, riparian forest, and urbanization were each significantly correlated with black spot incidence for at least four fish species. These data suggest that in southern Ontario streams, habitat degradation accompanying agricultural and urban development is associated with increased incidence of black spot in a variety of fish species. The data also support informed and cautious use of measures of black spot incidence in multivariate indices of stream health, such as the index of biotic integrity.