The results of deoxyribonucleic acid homology experiments with the type strains of Azospirillum lipoferum, Azospirillum brasilense, and Azospirillum amazonense, and 19 additional strains of A. amazonense, confirmed that A. amazonense is a distinct new species. The description of the genus Azospirillum is emended to accommodate A. amazonense. Ribosomal RNA (rRNA) homology studies indicated 90 to 96% homology between Azospirillum lipoferum and Azospirillum brasilense, and 64 to 70% homology between these species and Azospirillum amazonense. This supports the inclusion of these three species in the genus Azospirillum. In contrast, "Azospirillum seropedicae" had very little homology with the other Azospirillum species (<22% RNA homology) and should not be considered a member of the genus. The taxonomic placement of "A. seropedicae" is uncertain. The nearest relatives of the genus Azospirillum were Aquaspirillum itersonii and Rhodospirillum rubrum (>65% RNA homology); Gluconobacter oxydans and Beijerinckia indica exhibited 30 to 60% RNA homology with Azospirillum species. Deoxyribonucleic acid (DNA) studies indicated that Conglomeromonas largomobilis subsp. largomobilis was related to Azospirillum lipoferum by >45% DNA homology and by 99% RNA homology; moreover, it was found to be a microaerophilic nitrogen fixer. Thus C. largomobilis subsp. largomobilis is a subjective synonym of A. lipoferum. In contrast, DNA homology studies indicated that Conglomeromonas largomobilis subspecies parooensis was unrelated to C. largomobilis or A. lipoferum, or to any other species tested, and its taxonomic position is uncertain. Several strains of azospirilla which form unique star-shaped colonies were identified as A. lipoferum by DNA homology. / Master of Science