Effects of hexavalent chromium on the life cycle of Chironomus riparius were examined in the laboratory, using sublethal (0.5 mg/1) and 40% lethal (2.0 mg/1) concentrations. Chironomus egg masses were introduced into both control and chromium-treated 21 1 aquaria. Larvae were subsampled at 5-day intervals, measured for length, and weighed to generate growth curves. Emerging adults were allowed to mate and oviposit. Twenty female pupae were collected from control and treatment (0.5 mg/1) populations within 12 hours of pupation. Their bodies were analyzed for lipid content by calculating loss in dry weight after a chloroform-methanol extraction. Chromium body burdens were measured in both larvae and adults from treatment (2.0 mg/1) and control populations. Effects of chromium on hatching rates of egg masses were studied. Growth was stimulated at both lethal and sublethal concentrations of chromium. Lipid content of female pupae was significantly higher in treatment populations than in controls. Both stimulated growth and increased lipid storage may be the results of metal activation of enzymes involved in energy processing and storage in the insect. Reproductive success, as measured by numbers of offspring per adult female, declined with increasing chromium concentrations. Chromium concentrations in adult males and larvae were significantly greater for treatment populations. Metal body burdens may have become concentrated in eggs or sperm of Chironomus and caused the observed decrease in reproductive success. No direct toxicity of chromium on egg masses was observed at concentrations up to 5 mg/1. / M.S.