Diflubenzuron (WP 25%) was tested in small artificial streamlets against Simulium vittatum Zetterstedt and a mixed population of Simulium spp. At 0.2, 0.1 and 0.02 ppm A1 for 1 h of exposure. The granular formulation (0.5%) was tested against a mixed population at 0.04 ppm based on 1 h exposure. The 0.2 and 0.1 ppm gave about 95 + % control of exposed larvae of S. vittatum 30 m below treatment point. The mixed population responded with 100 and 76% mortality (200 m below treatment) of exposed larvae to the 0.2 and 0.02 ppm concn respectively. The granular formulation produced very little mortality in the mixed population. At 0.2 ppm/1 h diflubenzuron when tested against a mixed population [S. bivittatum Malloch (86%) and S. argus Williston (14%) in a moderate-size stream, produced complete mortality of the exposed larvae for the entire length of the stream (4.3 km). Within 3.5 weeks the population rebounded to 56% of the pretreatment numbers. Seven weeks after treatment the population was 64% of the pretreatment numbers.