A facultatively chemolithotrophic thiocyanate-degrading bacterium, strain THI 011T, which was previously isolated from activated sludge and tentatively named Thiobacillus sp., was studied taxonomically and phylogenetically. This bacterium utilizes thiocyanate as sole energy source and the specific growth rate for chemolithoautotrophic growth with thiocyanate was 0.059 h-1. Molecular phylogenetic relationships of strain THI 011T to Thiobacillus versutus and members of the genus Paracoccus were elucidated by comparing 16S rRNA gene sequences. Binary sequence comparisons showed that strain THI 011T was most related to Paracoccus aminophilus, at a similarity level of 97.0%, and T. versutus was most similar to Paracoccus denitrificans, at a level of 99.1%. A neighbour-joining phylogenetic tree showed that strain THI 011T formed a cluster together with T. versutus and known species of the genus Paracoccus within the alpha-3 subclass of the Proteobacteria. DNA-DNA hybridization assays and phenotypic studies indicated that strain THI 011T differed from T. versutus and known species of the genus Paracoccus. On the basis of these results, we propose to classify strain THI 011T into a new species of the genus Paracoccus with the name Paracoccus thiocyanatus sp. nov. We also propose to transfer T. versutus to the genus Paracoccus and present an emended description of the genus.