Perchlorate and hexavalent chromium are two common anthropogenic water pollutants. The effects of these environmental contaminants on insects and ecosytems are not well understood. In this study, we compared the chronic (14 day) mortality of larval mosquitoes reared in water contaminated with perchlorate or hexavalent chromium to the acute mortality of fourth-instar mosquitoes exposed to perchlorate or hexavalent chromium for 24 or 48 hours. The LC50 values for chronic exposure to perchlorate or hexavalent chromium were approximately 90 ppm and 800 ppb, respectively. The LC50 values for acute toxicity were significantly higher. Thus, not only are these common pollutants affecting food webs, but the typical acute toxicity technique greatly underestimates concentrations causing environmental effects.