Larvae of 4 species of black flies, Prosimulium mixtum, Simulium vittatum, S. decorum and Stegopterna mutata, were exposed in the laboratory to cercariae of the digenean Plagiorchis noblei in flowing water. Prevalence and intensity of infection of all species varied directly with exposure intensity and decreased with increasing water velocity; prevalence increased with larval size. Infection levels for P. mixtum were higher than for the 3 other species. Mortality among infected larvae of all species was 3 times that of controls. Exposure of black fly larvae to P. noblei cercariae may adversely affect their survival.