A comparative analysis of 16S ribosomal DNA sequences from 18 strains belonging to different species of the genus Chlorobium has been made in order to investigate their phylogenetic relationships. All studied strains with brown pigmentation, belonging to species C. phaeobacteroides and C. phaeovibrioides, were clustered together and separated from green species. Most of the C. limicola strains formed a consistent group and were clustered next to C. tepidum. But, despite their identical morphology, C. vibrioforme strain 6030 (= DSM 260T) and C. vibrioforme strain 8327 (= DSM 263) were clearly differentiated on the basis of their 16S rDNA sequence. On the other hand, morphologically distinctive strains of C. chlorovibrioides, C. vibrioforme and C. limicola presented sequences with a high degree of similarity. Since morphological characters do not seem to be sufficient to adequately classify this group, these findings help to set up the basis for a revision of the taxonomy of the genus Chlorobium.