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The respiratory activity of three aquatic animals, selected from different pollution index classifications, was examined in the presence of sublethal concentrations of Dibrom, a commonly used, broad spectrum, organophosphate insecticide. The oxygen consumption of Hydroperla crosbyi (Needham and Claassen), Corydalus cornutus L. and Notemigonus crysoleucas (Mitchill) was measured in a specially designed flowing-water respirometer, housed in an environmental chamber. The acute toxicity of Dibrom, measured as 24-hr LC50's for these animals, was compared in both static and flowing-water bioassays and found to be significantly more toxic to both the hell-grammites and stoneflies in the flowing system (LC50: 6.8 ppm and 11.4 ppb, respectively) than in the static bioassay (LC50:9.5 ppm and 16.0 ppb, respectively). There was a marked difference in the toxicity of Dibrom to golden shiners between flowing and static systems but this was not statistically significant at the 95% confidence level. Sublethal Dibrom concentrations significantly affected oxygen consumption (QO2) in all test animals, and reduced their tolerance to low oxygen tensions. The ability of the golden shiner to tolerate low oxygen tensions was reduced by 50% by exposure to 5 ppm Dibrom concentrations. Stonefly body undulations and hellgrammite gill beats were increased by exposure to sublethal toxicant levels in the flowing system. The flowing-water respirometer offers an approach to simulate the lotic environment in bioassays and laboratory assessments of the effects of pollutants on respiration and other organism responses. A standardization of technique is necessary to avoid the lengthy description of operation and methods which is now necessary before results can be reported.