16S ribosomal RNA gene sequences from seven strains of Aquaspirillum peregrinum, Aqu. itersonii, Aqu. Polymorphum, and Oceanospirillum pusillum were compared with homologous sequences from other members of helical-shaped bacteria. The bootstrapped neighbor-joining tree, inferred from 887 aligned sites, placed the spirillum taxa assigned to Aquaspirillum, Oceanospirillum, Azospirillum, Magnetospirillum, Rhodospirillum, and Rhodocista of the Proteobacteria in seven clusters of α Proteobacteria separately from other shapes of bacteria. Aqu. Peregrinum and Aqu. Itersonii grouped together in 88% bootstrap support. They were more related to Rhodospirillum rubrum and Rsp. Photometricum than Aqu. polymorphum. Aqu. Polymorphum was close to Magnetospirillum gryphiswaldense, Mag. magnetotacticum, Rsp. fulvum, and Rsp. Molischianum, and more close to Mag. gryphiswaldense. Oce. Pusillum was not related to other spirillum taxa and was placed in a separate branch. Rhodocista was very closely related to Azospirillum. Photosynthesis and magnetotaxis, as phenotypic characters, were not important in the classification of helical bacteria.