Water velocity, substrate placement and the length of exposure time influenced the number ofProsimulium mixtum/fuscumSyme and Davies andCnephia dacotensisDyar and Shannon larvae attaching to white ceramic tiles and clear plastic tapes in the field.P. mixtum/fuscumreadily attached to either substrate whereasC. dacotensisdemonstrated a preference for white ceramic tiles. In an artificial stream, both species preferred specific velocity ranges although they did not increase their relocation behavior when exposed to varying depths of 2 to 30 cm and temperatures of 2° to 30°C.P. mixtum/fuscumincreased relocation behavior in the absence of light.