PAUL, R. W., JR., KUHN, D. L., PLAFKIN, J. L., CAIRNS, J., JR. & CROXDALE, J. G. 1977. Evaluation of natural and artificial substate colonization by scanning electron microscopy. Trans. Amer. Micros. Soc., 96: 506-519. Progressive microbial colonization of natural (sycamore leaf) and artificial (polyurethane foam) substrate material in lotic and lentic systems was studied by scanning electron microscopy. The organisms which participated in colonization showed substrate preferences and aquatic system variation. The rate of colonization was dependent not only on the aquatic system, but also on the type of substrate. Protozoa were not found on either substrate, indicating that they are lost during specimen preparation. Fungi and bacteria were principal colonizers on leaf substrates, while algae were predominate polyurethane foam invaders. In each case, colonization served to structurally modify the substrate, rendering it a more suitable habitat which was nutritionally capable of supporting other organisms.