The bootstrapped 16S rDNA sequence-based neighbor-joining phylogeny has suggested that the marine species of the genus Agrobacterium have no relation to the terrestrial Agrobacterium species. Agrobacterium atlanticum IAM 14463^T (a superscript T=type strain), Agrobacterium ferrugineum IAM 12616^T, Agrobacterium gelatinovorum IAM 12617^T, Agrobacterium meteori IAM 14464^T, Agrobacterium stellulatum IAM 12621^T and IAM 12614, and the invalidly published marine species “Agrobacterium kieliense” IAM 12618 occupy an independent position in the α-subclass of the Proteobacteria. Based on 16S rDNA sequencing and on chemotaxonomic, morphological, and physiological studies, we propose the transfer of A. atlanticum, A. gelatinovorum, and Roseobacter algicola to the genus Ruegeria gen. nov. as Ruegeria atlantica comb. nov., Ruegeria gelatinovora comb. nov., and Ruegeria algicola comb. nov., respectively; of strains of A. stellulatum to the genus Stappia gen. nov. as Stappia stellulata comb. nov. and Stappia aggregata sp. nov., nom. rev., respectively; and of “A. kieliense” to the genus Ahrensia gen. nov. as Ahrensia kieliense sp. nov., nom. rev. Agrobacterium meteori is assigned to be a synonym of A. atlanticum.