DDT in acetone solution produces a high mortality among Simulium damnosum larvae at concentrations much lower than normally used in field control. After a 1-hour exposure the LD50 is 0.005 parts per million on a 24-hour basis, and 0.002 ppm. on a 48-hour basis. Mortality approaches 100 per cent 48 hours after a 1-hour exposure to 0.01 ppm. A 30 per cent emulsion concentrate formulation tested was rather less effective, under laboratory conditions, than DDT in acetone.