Laboratory test conducted on lab reared black flies, Simulium verecundum and field collected Cnephia ornithophilia, C. mutata, S. vittatum and Prosimulium mixtum demonstrate that Simulium larvae are susceptible to a mosquito pathogenic strain of Bacillus thuringiensis. For exposure times of 30 minutes and 24 hours LC50's were between 4.4x102 and 5.3x103 viable cells/ml. One minute exposures of S. verecundum to 5x104 and 1x105 cells/ml resulted in 87 and 93% mortality respectively.