Diflubenzuron{sup 1} (Dimilin{reg_sign}) was used on over 7 million acres in the U.S. in 1990 to control forest pests, particularly the gypsy moth. This chitin synthesis inhibitor affects insects and other anthropods. It is a restricted use pesticide due to its nontarget effects on aquatic macroinvertebrates. The effects of a single aerial application on nontarget aquatic macroinvertebrate communities were reviewed by Eisler (1992). Crustacea and immature insects (especially the true flies, mosquitoes, midges and black flies) are the most sensitive nontarget aquatic organisms to diflubenzuron. Diflubenzuron, N-[[4-(chlorophenyl)amino]carbonyl]-2,6-difluorobenzamide, is not the only mortality factor aquatic organisms face from human pollution. Acid deposition is a frequent stress factor in freshwater habitats in the Northeast USA. Acidic pulses can drop vernal pools (e.g., temporary, springtime, snowmelt pools) to pH levels below 3.0. Aquatic invertebrates vary in their tolerance to acidification. Reduced pH completely eliminates some species. A combination of stress factors could lead to synergistic effects, over and above the impact seen with a single stressor. The purpose of this study was to determine if there are synergist effects of diflubenzuron and lowered pH on the mortality of a nontarget aquatic organism. 11 refs., 1 fig.