When the type strains and other strains of the six currently defined species of the genus Ectothiorhodospira were examined by DNA-DNA reassociation and RFLP of 16S/23S rDNA (ribotype), only four genospecies could be found. The possibility of defining taxonomically meaningful species corresponding to these four genospecies was investigated by combining DNA relatedness and ribotype data with other genotypic and phenotypic characters already described in the literature, an approach known as polyphasic taxonomy. Following this comparison, the type strain and another strain of Ectothiorhodospira vacuolata were found to be very similar to the type strain of Ectothiorhodospira shaposhnikovii and have been transferred to this latter species. Also, the type strain of Ectothiorhodospira marismortui and another previously unidentified strain were found to be very similar to the type strain of Ectothiorhodospira mobilis and have been transferred to this latter species. Due to the limited degree of reciprocal DNA relatedness, strains belonging either to Ectothiorhodospira marina or to Ectothiorhodospira haloalkaliphila are still considered as belonging to separate species, even though they show a remarkable phenotypic similarity. This revision has led to the delineation of only four species in the genus Ectothiorhodospira, namely E. mobilis, E. shaposhnikovii, E. marina and E. haloalkaliphila. E. vacuolata is recognized as a junior synonym of E. shaposhnikovii and E. marismortui as a junior synonym of E. mobilis.