The structural assimilative capacity (ability to maintain biological integrity under stress) of protozoan communities from nine lakes in the area of the University of Michigan Biological Station, Pellston, Michigan, and six stations at Smith Mountain Lake, Virginia, were studied (1) to determine if communities from lakes of differing trophic state differ in their ability to assimilate various amounts of copper sulfate, and (2) to explore the possible influence of average density of individuals and/or qualitative differences in the types of species present on any observed differences in assimilative capacity. In both the northern Michigan and Smith Mountain Lake studies, a trend in response was demonstrated along the eutrophic-oligotrophic gradient; eutrophic communities had a greater structural assimilative capacity than did oligotrophic communities. Both mean species density and community composition appear to be important factors in the ability to maintain structural integrity.