This study was designed to determine: (1) Whether a sublethal dose of CuSO4 would affect the colonization rate of freshwater protozoan communities established on polyurethane foam units, and (2) if a correlation exists between the length of initial colonization of an artificial substrate epicenter in the lake and the extent of the toxicant’s effects on the colonization process in the laboratory. Observed colonization rates for the sytems under stress were compared to those of the control systems at the same stage of community development. A sublethal dose of copper sulfate significantly decreased the colonization rate of both the mature and immature communities (P<0.05); however, the effects were greater on the immature systems.