Glutathione S-transferases constitute a large family of enzymes which catalyze the addition of glutathione to endogenous or xenobiotic, often toxic electrophilic chemicals. Eukaryotic glutathione S-transferases usually promote the inactivation, degradation or excretion of a wide range of compounds by formation of the corresponding glutathione conjugates. In bacteria, by contrast, the few glutathione S-transferases for which substrates are known, such as dichloromethane dehalogenase, 1,2-dichloroepoxyethane epoxidase and tetrachlorohydroquinone reductase, are catabolic enzymes with an essential role for growth on recalcitrant chemicals. Glutathione S-transferase genes have also been found in bacterial operons and gene clusters involved in the degradation of aromatic compounds. Information from bacterial genome sequencing projects now suggests that glutathione S-transferases are present in large numbers in proteobacteria. In particular, the genomes of three Pseudomonas species each include at least ten different glutathione S-transferase genes. Several of the corresponding proteins define new classes of the glutathione S-transferase family and may also have novel functions that remain to be elucidated.