Wastewater from tanneries contains high concentrations of organic matter, chromium, nitrogen, and sulfur compounds. In this study, an artificial wetland is is used as the tertiary treatment in a tannery in León Gto., México. It consists of three subplots with an area of about 450 m2. Two subplots were planted with Typha sp. and the third with Scirpus americanus. Geochemical analyses along the flowpath of the wetland show that contaminants were effectively attenuated. The most probable number technique was used to determine rhizospheric microbial populations involved in the sulfur cycle and suggested that there were 104-10(6) cells g(-1) sediment of sulfate-reducing bacteria and 10(2)-10(5) of sulfur-oxidizing bacteria (SOB). Representatives of SOB were isolated on media containing thiosulfate. Phylogenetic analysis of 16S rRNA of SOB isolates shows that they belong to the genera Acinetobacter, Alcaligenes, Ochrobactrum, and Pseudomonas. Most of the isolates are organotrophic and can oxidize reduced sulfur compounds such as elemental sulfur or thiosulfate, accumulating thiosulfate, or tetrathionate during growth. All isolates can use reduced-sulfur compounds as their sole sulfur source and some can use nitrate as an electron acceptor to grow anaerobically. Our results illustrate the relevance of SOB in the functioning of the wetland constructed for tannery wastewater remediation.