The susceptibility of Simulium vittatum to varying concentrations of Bacillus thuringiensis var. israelensis (Abbott Lab. #6406-125 with an ITU/mg of 400-600) after 15,30,60.90 120 min and 24 hr of exposure was studied. A sharp decrease in LC50 (0.81 to 0.32 ppm) and LC90 (1.71 to 0.86 ppm) concentration values occurred when exposure times were increased from 30 to 60 min. Change in LC50 and 90 concentration requirements was minimal when exposure times were expanded beyond 60 min.