The morphological, physiological, and nutritional characteristics and the deoxyribonucleic acid (DNA) base compositions of 39 strains of SpirilZum (including the available type strains) were determined. A uniform methodology provided a basis for comparison of the strains. The results, together with previous serological data and with previous results for two strains of Spirillum volutans, indicate that the present genus Spirillum should be divided into three genera, with the original name being restricted to obligately microaerophilic freshwater forms having a large cell diameter and a DNA base composition of 36 to 38 mol % guanine + cytosine (G + C). This genus would contain at present only one species, the type species, S . uolutans. The generic name Aquaspirillum is proposed for the aerobic, freshwater forms having a DNA base composition of 49 to 65 mol % G + C. Although Spirillum Ehrenberg 1832 is one of significance, except for Spirillum minor, the the oldest bacterial generic names and although causative agent of one of the forms of rat-bite spirilla are widespread in freshwater and marine fever in man (3, 8, 29); (ii) difficulty of environments, characterization of this group isolation (7,28,45,49); and (iii) past has not been extensive. This may be attributed difficulties in preservation of cultures, with t o the following factors: (i) lack of medical only continual transfer, the recently developed Contribution no. 3032 of the Woods Hole Oceanographic Institution, Woods Hole, Mass.