Sixty-one strains of the root-associated nitrogen fixer <u>Spirillum lipoferum</u> exhibited a similar morphology (slightly-curved to S-shaped cells having a cell diameter of 1.0 μm) and had a DNA base composition of 69 - 71 mol% G + C (Tm). When grown in broth the cells had a single polar flagellum, but when grown on agar at 30°C they also formed numerous lateral flagella of shorter wavelength. DNA homology experiments using a membrane filter competition method indicated the occurrence of two distinct but related homology groups: 46 strains were in Group I and 15 strains were in Group II. Group I strains gave DNA homology values of 67% when tested against reference strain Sp 7, whereas Group II strains gave 28 - 46%. Group II strains gave homology values of 70% when tested against reference strain 59b, whereas Group I strains gave 30 - 52%. Group II strains were distinguished by their acid reaction with glucose, a weak fermentative ability, a requirement for biotin, and formation of wider, longer, S-shaped to spirillum-shaped cells when transferred to nitrogen-deficient malate medium. Pigmentation, catalase activity, and denitrifying ability varied considerably. All strains could grow anaerobically in peptone media with nitrate as the electron acceptor. The results indicate that two species exist within the <u>S. lipoferum</u> group, and the characteristics of the organisms suggest that they should be assigned to a new genus, <u>Azospirillum</u>. Strains belonging to Group II are named <u>A. lipoferum</u>, while those belonging to Group I are named <u>A. brasilense</u>. / Master of Science