Phylogenetic analysis based on 16S rRNA gene sequences showed that Aquaspirillum itersonii and Aquaspirillum peregrinum form distinct phylogenetic lineages within the Alphaproteobacteria, whereas Aquaspirillum serpens, the type species of the genus Aquaspirillum, belongs to the Betaproteobacteria. A. itersonii and A. peregrinum exhibited 16S rRNA gene sequence similarity values of 82.0-82.4 % to the type strain of A. serpens and of 91.8-92.0 % to each other. A. itersonii and A. peregrinum were clearly distinguishable from A. serpens by differences in ubiquinone types and fatty acid profiles. A. itersonii subsp. itersonii LMG 4337(T) and A. itersonii subsp. nipponicum LMG 7370(T) contained Q-10 as the predominant ubiquinone, and A. peregrinum subsp. peregrinum LMG 4340(T) and A. peregrinum subsp. integrum LMG 5407(T) contained Q-9 as the predominant ubiquinone, whereas A. serpens LMG 3734(T) had Q-8 as the predominant ubiquinone. A. itersonii and A. peregrinum were also distinguishable from A. serpens by some differences in the fatty acid composition, including major fatty acids and hydroxy fatty acids. On the basis of these data, A. itersonii and A. peregrinum should be reclassified into two novel genera and species, for which the names Novispirillum itersonii gen. nov., comb. nov. and Insolitispirillum peregrinum gen. nov., comb. nov., respectively, are proposed.