At the abandoned mining site of Carnoulès (Gard, France), weathering of the arsenopyrite rich tailings leads to the formation of acidic effluents heavily loaded with arsenite (As(III)). However, further downstream, the As(III) concentration decreases while the sediments are richer in arsenate (As(V)). A Thiomonas sp. able to oxidize arsenite to arsenate has been isolated and characterized. The aoxA and aoxB genes encoding the two subunits of this enzyme belong to an operon. Analysis of the genome sequence (Genoscope, Ivry, France) shows that this operon encodes also two cytochromes c, which could be the physiological partners of the arsenite oxidase, and a transcriptional regulator belonging to the metalloregulator ArsR/SmtB family, which could control the expression of the aox operon. The expression of this operon is higher in the presence than in the absence of As(III) and appears also to be repressed in the presence of thiosulfate, a more energetic substrate.