The nucleotide sequence of the 3-chlorobenzoate 3,4-dioxygenase genes, designated cbaAB, from the transposon Tn5271 was determined. FMN-isoalloxazine and NAD-ribose-binding domains and the orientation of these domains is conserved in all known class IA reductases. These results support the hypothesis that alternative fusions of the electron transfer modules of the reductases arose early in the divergence of oxygenase systems. The over-riding evolutionary constraint acting on the divergence of the class IA oxygenases would appear to be the requirement for a carboxyl group para to the site of oxygen insertion into the aromatic ring.