Protozoan communities established in plastic troughs through which Douglas Lake (Michigan) water was pumped were exposed for twenty-four hours to either 24 ppm of Zn+2 or 24 ppm Cu+2. The number of species and approximate density of protozoans in each trough were determined before the zinc or copper was introduced, at the end of the twenty-four hours exposure period, and at each twenty-four hours period after that for either 120 or 144 hours. These results were then compared to those obtained for controls receiving identical treatment except for the omission of heavy metal. A subsidiary test was carried out by exposing a protozoan population in a plastic trough to 24 ppm of zinc with an exposure period of one hour. The results for the 24 ppm Cu+2 test solution show a reduction from 46 to 7 species at the end of the twenty-four hours exposure with a partial recovery to 14 species 144 hours after the test began. The results for the 24 ppm Zn+2 test solution show a reduction in species from 35 to 11 after twenty-four hours exposure with a comparatively rapid recovery to 20 species at 48 hours, 30 at 72, 32 at 96, and 34 at 120. Control species number varied between 32 and 36 during the zinc tests. Copper test controls were similar, varying from 37 to 48. Density changes were not as striking as the reduction in number of species, although more detailed studies may reveal significant changes. The effects of the one hour exposure to zinc were negligible. These preliminary experiments indicate that the residual effects of copper seem to be considerably greater than the residual effects of zinc.