Ninety-seven strains of budding bacteria originating from various aquatic habitats and morphologically resembling planctomycetes were investigated taxonomically. Taxonomic differentiation was based on DNA–DNA hybridization, physiological properties and chemotaxonomic tests. Nineteen hybridization groups, containing 79 of the tested strains, were established. Eighteen strains, however, did not fit into any of these groups. Rhodopirellula baltica gen. nov., sp. nov. is described, with strain SH 1T (=IFAM 1310T=DSM 10527T=NCIMB 13988T) as the type strain. Pirellula marina is transferred to the genus Blastopirellula gen. nov. as Blastopirellula marina comb. nov., with strain SH 106T (=IFAM 1313T=DSM 3645T=ATCC 49069T) as the type strain. An emended description of the genus Pirellula is also provided. Differentiation between R. baltica, B. marina and Pirellula staleyi was achieved by the integration of morphological, physiological, chemotaxonomic and genetic characteristics.