Forty-four strains (including 20 type strains) of spirilla, have been subjected to a comprehensive nutritional characterization. A total of 67 different organic compounds were tested for their ability to serve as sources of carbon and energy. A total of 30 different compounds were tested for their ability to serve as sole nitrogen sources. Other characters that were studied included: urease production; NaCl tolerance of fresh-water Spirillum strains; effects of calcium ions on growth of Spirillum strains; production of fluorescent pigments; enzyme assays of key enzymes of carbohydrate catabolism in P. lunatum, A. itersonii, and A. peregrinum; and the effects of various inhibitors on glucose-6-phosphate dehydrogenase activity from P. lunatum. The resultant data, plus that of Wells (1970) and McElroy (1970), have been used to divide genus Spirillum into four genera (Aquaspirillum, Spirillum, Oceanospirillum, and Pseudospirillum), each possessing its own unique characteristics. Diagnostic keys are presented for the four genera and also the species of each genus. Each of the four genera were compared with other related genera of bacteria. The species O. minutulum (formally Spirillum minutulum) and O. halophilum (formally Spirillum halophilum) were combined into a single species (O. minutulum) on the basis of their morphological, physiological, nutritional, serological, and DNA base composition similarities. The two species O. linum (formally Spirillum linum) and O. atlanticum (formally Spirillum atlanticum), were also combined into a single species (O. atlanticum) because of their similarities. Previously unnamed strains 101, 102, and 206 were proposed as a new species, O. maris. With regard to the genus Aquaspirillum, one new species was proposed, A. mobile (formally unnamed strains 103 and 104). Characteristic enzymes of the Entner-Doudoroff Pathway were found in P. lunatum and A. itersonii. Key enzymes of the EmbdenMeyerhof-Parnas Pathway were detected in A. itersonii and A. peregrinum. The characteristic enzymes of the hexose monophosphate pentose pathway were not detected in any of the above species. Glucose-6- phosphate dehydrogenase from P. lunatum was found to be inhibited by ATP and also to be non-specific for NAD⁺ or NADP⁺. / Ph. D.