The distribution of limnetic crustacean zooplankton species and species associations in 47 industrially acidified lakes of the La Cloche Mountains, Ontario, are examined. pH, which ranged from 3.8 to 7.0, and to a lesser extent lake area and depth are the major determinants of the structure of these communities. Mesocyclops edax, Cyclops bicuspidatus thomasi, Diaptomus minutus, Holopedium gibberum, Diaphanosoma leuchtenbergianum, and Bosmina sp. were common species which occurred over the entire pH range. Tropocyclops prasinus mexicanus, Epischura lacustris, Diaptomus oregonensis, Leptodora kindtii, Daphnia galeata mendotae, D. retrocurva, D. ambigua, and D. longiremis plus some other rare species making a total of 64% of all species found never or rarely occurred below pH 5.0. Polyphemus pediculus, Daphnia catawba, and D. pulicaria were found primarily in lakes with low pH. An increase in the complexity of the zooplankton communities with increasing pH was evident. Above pH 5.0 the communities contained 9–16 species with three or four dominant species; in lakes with pH 5.0 or less the communities comprised one to seven species with only one or two dominants. Compared with relatively unspoiled lakes of similar morphometry in northwestern Ontario (ELA lakes) the La Cloche lakes have unusually simple zooplankton associations, particularly the very acidic lakes. Recurrent group analysis of the La Cloche and ELA lakes indicated that they were both characterized by a single major recurrent group of six species, five of which were common to the two areas. In the La Cloche lakes the group of six species occurred most frequently in lakes with pH above 5.0 although it maintained its integrity even down to pH 5.0. It is concluded that pH has a great effect on these zooplankton communities, primarily in lakes with pH below 5.0 where many species are completely eliminated and even tolerant species become progressively rarer until in some lakes a single species (D. minutus) remains.