Eleven pure cultures of Sphaerotilus discophorus were isolated from streams, rivers and lakes by appropriate enrichment culture techniques. The morphology of the colonies, filaments and individual cells was investigated in detail. The carbon, nitrogen, mineral and growth factor requirements of S. discophorus were determined. The effects of pH, temperature and oxygen on growth were established. The extent to which iron and manganese are accumulated in the sheaths of S. discophorus was determined by quantitative analyses. The oxidative patterns of S. discophorus with carbohydrates, amino acids and TCA cycle intermediates were determined and the occurrence of a high degree of oxidative assimilation was established. Large amounts of poly-beta-hydroxybutyric acid were found in the cells of both S. discophorus and S. natans. Preliminary investigations were also made on the problem of chemoautotrophy in Sphaerotilus. (Author)