The gene coding for an aerobic azoreductase was cloned from Pigmentiphaga kullae K24, which is able to grow with the carboxylated azo compound 1-(4'-carboxyphenylazo)-4-naphthol (carboxy-Orange 1) as sole source of carbon and energy. The gene encoded a protein with a molecular weight of 20,557 Da, with a conserved putative NAD(P)H-binding site in the amino-terminal region. The deduced amino acid sequence showed no further significant sequence homologies to previously studied aerobic azoreductases. The azoreductase was heterologously expressed in Escherichia coli and shown to convert the sulfonated azo dye Orange I and furthermore Magneson II [4-(4-nitrophenylazo)-1-naphthol].