The zooplankton and zoobenthos were investigated in six lakes in the Sudbury area of Ontario in which pH values lay between 4.05 and 7.15. Zooplankton showed a significant reduction in species and numbers at the lower pH levels; the Rotifera showed the greatest changes. Zooplankton diversity index (DI) declined sharply below pH 5.3. Percentage similarity of community (PSc) and coefficient of community (CC) values were highest between lakes closest in pH, reflecting progressive changes in species composition.Significant relationships were found between the zooplankton standing crop and chlorophyll a concentrations and the zooplankton standing crop and areal primary productivity. Dry weights for Holopedium gibberum and Diaptomus minutus were lowest in the most acidic lakes. No significant relationship between body length or egg ratio, and lake pH were found for D. minutus.The zoobenthos was represented by a variety of taxa; however, only genera belonging to the Oligochaeta and Tendipedidae were represented in all study lakes. Zoobenthos DI showed only a slight decrease below a pH of 4.9. PSc and CC values for the zoobenthos displayed similar though not as distinct patterns as those found in the plankton.