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. Colonization studies of both natural and artificial substrates are becoming increasingly common for academic reasons and as pollution assessment techniques. Techniques involving substrate introduction into aquatic systems have l)een refined and adapted for studying decomposition, colonization, energy flow, and organism collection. Cooke (1956), Sladeckova (1962), and Wetzel (1964) have contributed extensive reviews of artificial substrate use; Cairns et al. (1969) have shown polyurethane foam to be an effective sampling device for protozoa. Kaushik & Hynes (1968, 1971), Petersen & Cummins (1974), and others have demonstrated the value of natural substrate placement in energetic evaluations. IHowever, the visual aspects of the colonization process are not customarily included in these studies. Early investigations on microbial colonization and succession on glass slides were conducted by Henrici (1933), Karsinkin (1934), and Gause (1936); later, on glass and plastic surfaces by Spoon & Burbanck (1967); and on plexiglass by Allen (1971) and Wetzel & Allen (1971). Although organisms collected by polyurethane foam substrates have received considerable attention, colonization and the means by which foam substrates support colonization have not been studied. Leaf material substrates in aquatic environments have also been of interest, due to the wide variety of microorganisms and invertebrates which they nutritionally support. Few authors, however, have presented direct visual evidence of leaf material colonization. Among those who have documented colonization of natural substrate material in freshwater environments by scanning electron microscopy (SEM) are Suberkropp & Klug (1974), who reported seasonal changes in leaf surface microorganisms of a small stream, and Paerl, who showed structural modification of detritus and commented on the nutrition of organisms colonizing organic particles in Lake Tahoe (Paerl, 1973, 1974; Paerl et al., 1975). Due to the 1This investigation was supported, in part, by grant E-[40]-1-[-4939] from ERDA. We are indebted to Dr. Rex L. Lowe for his reading of a rough draf...