The objectives of this study were to examine the colonization process for diatoms and protozoans in a variety of Michigan lakes and in a southwest Virginia pond, and to examine artificial substrate colonization during the first day of immersion. We hypothesized that diatom and protozoan species accrual would be similar because the organisms are approximately the same size and share similar ecological conditions. Polyurethane foam substrates were placed in the littoral zone of these lakes, and species accrual was monitored after 1,3,7,14, and 21 days of exposure. The species-time data were fitted to the MacArthur-Wilson equilibrium model using non-linear least squares regression. Protozoan species accrual fit the model in most cases; however, the diatom data did not. Further evaluations of species accrual by short-term (<1 day) exposure revealed a high number of diatom species in the water column. These results suggest that diatom species accrual on polyurethane foam artificial substrates does not follow MacArthur-Wilson predictions. Apparently, diatoms are present in the water column and do not traverse inhospitable terrain but are merely sampled by the substrates. Protozoan species accrual appears to follow predictions of the Mac-Arthur-Wilson model. Sampling and study methods must be selected carefully, even for closely related taxonomic groups.