Redundant primers were designed for the PCR amplification of DNA from chlorocatechol dioxygenase genes. These primers were used successfully to amplify 270- to 279-bp fragments from a variety of 2,4-dichlorophenoxyacetate- and cholorobenzoate-degrading strains, including species of Sphingomonas. Three groups of closely related sequences were amplified: one from chlorobenzoate degraders that was 86% similar to the amino acid sequence of the protein coded by the tfdC gene of Ralstonia eutropha JMP134 (pJP4), a second from Sphingomonas strains that was 70% similar to this amino acid sequence, and a third from diverse 2,4-D degraders that showed only 53% similarity to the product coded by tfdC from pJP4 but 88-100% similarity to the product of the tfdC gene of the plasmid pEST4011 from a Pseudomonas putida strain. The primers should be useful in further study of this gene and in tracking a variety of degraders of chloroaromatic compounds in natural systems.