Fresh-water protozoan communities inhabiting plastic troughs through which unfiltered Douglas Lake (Michigan) water flowed were exposed to pH and temperature shocks. The time required for the community to achieve a species diversity (i.e., number of species) comparable to that existing before the shock as well as the control diversity was studied. Diversity was the only charactersitic studied. Others, such as numbers of individuals and nutritive relationships, though important, were beyond the scope of this study. The two most important conclusions to be drawn from this preliminary study are (1) the magnitude or intensity of the shock seems to be more important in reducing protozoan species diversity than its duration and (2) restoration of protozoan species diversity may require only a few hours if the shock is mild, but as much as five or six days for severe shocks. The latter is probably true only of pH, temperature and other shocks where there is no toxicant residual left in either substrate or organisms.