Most data in the broad field of protozoan ecology, even if one restricts the subject to consideration of free-living species, are scattered through the literature and are generally of an unsophisticated nature. Review articles are few in number and/or out of date. In view of their high adaptability, their ubiquitousness, their numerousness, and their short generation time, it is perhaps surprising that protozoan forms have not been subjected to more study, both intensive and extensive in kind. The recognition in recent years of the possible role(s) played by the Protozoa in problems of pollution has caused renewed interest in many of these microorganisms. The inherent difficulties of proper taxonomic identification of the species under study and of more refined methods for carrying out desirable laboratory experimentation can be overcome, but time, motivation, and technical innovativeness are required. Because protozoan species are the ideal organisms to be investigated in many environmental situations, the field of protozoan ecology should develop significantly in the years ahead.