The genes for adenosine-5'-phosphosulfate (APS) reductase, aprBA, and sirohaem sulf ite reductase, dsrAB, from the sulfur-oxidizing phototrophic bacterium Chmmatium winosum strain D (DSMZ 1803 were cloned and sequenced. Statistically significant sequence similarities and similar physicochemical properties suggest that the aprBA and dsrAB gene products from Chr. winosum are true homologues of their counterparts from the sulfatereducing chemotrophic archaeon Amhaeoglobus filgidus and the sulfatereducing chemotrophic bacterium Desulfowibrio wlgaris. Evidence for the proposed duplication of a common ancestor of the dsrAB genes is provided. Phylogenetic analyses revealed a greater evolutionary distance between the enzymes from Chr. winosum and D. vulgaris than between those from A. fulgidus and D. wlgaris. The data reported in this study are most consistent with the concept of common ancestral protogenotic genes both for dissimilatory sirohaem sulfite reductases and for APS reductases. The aprA gene was demonstrated to be a suitable DNA probe for the identification of apr genes from organisms of different phylogenetic positions. PCR primers and conditions for the amplification of apr homologous regions are described.