In the course of a continuing investigation of the colonization of polyurethane foam blocks by fresh-water protozoans it seemed prudent to determine if (1) pore size and/or color of the polyurethane influence the colonization and (2) other materials suspended in the same way are colonized by the same species. In order to test (1) a series of polyurethane units of differing pore size and color were suspended in Douglas Lake, Michigan during the summer of 1972. To test (2) five natural substrates and one polyurethane unit were suspended in Douglas Lake during the summer of 1966. For (1) species determination were made every two weeks following immersion for a period of six weeks (three harvestings). For (2) species determinations were made at ten and twenty-two days following immersion. These preliminary results indicate that polyurethane pore size and color do not influence the number and kinds of species present but that certain types of natural substrates do probably because they are not inert.