A major problem of bacteriology has been the development of rapid, accurate techniques of isolating, identifying, and counting organisms in water, air, and food. The authors of this paper have undertaken a series of controlled experiments in the United States to develop and apply techniques employing a membrane filter for this purpose with water. The results reported promise a substantial reduction in the time, labor, and space required for specified phases of bacteriological analysis and, at the same time, they indicate a likelihood that these techniques will be more certain and precise in results than methods now in use. These experiments have a particular significance for the hygienic aspects of waterworks operations.