Diflubenzuron at 0.02 ppm AI/I h tested in the laboratory against late instars of field-collected larvae of Simulium vittatum S. argus, S. tescorum, S. aureum, and S. virgatum produced differential mortality in these species. S. vittatum was the least, while S. tescorum was the most susceptible species, responding with 59 and 88% mortality, respectively. Second and 3rd instars of S. vittatum were much more susceptible than the ultimate instars, suffering 3 times more mortality. There was a positive correlation (r = 0.99) between temperature and extent of mortality; larvae exposed to 0.02 ppm/I h at 10, 15, 19 and 24 C responded with 18, 35, 53 and 75% preimaginal mortality respectively. Diflubenzuron was shown to be active per Os. No contact activity was observed when larvae were exposed in static water. Rearing water pH and degree of turbidity and larval crowding were not shown to significantly influence the biocidal activity of diflubenzuron.