The microscopic flora and fauna exclusive of bacteria, of natural waters, prepared waters from water purification plants, and raw and treated sewage, offer a variety of information relative to those waters. The organisms frequently indicate the hardness or softness of the waters involved; the nature of the acidity, if acid; the region from which the water is derived; the degree of fertilization of the water; the source of tastes and odors; the degree of effectiveness of purification and filtration; the condition of a sewage treatment plant; and sometimes what specific trade wastes are present. Pollution can be classified. The report briefly reviews the use of such indicator species.