Trimethoprim resistance in the Enterobacteriaceae has been dominated by the spread of two genes encoding different dihydrofolate reductases (types IA and II). The original carriage of these genes was by transferable bacterial plasmids. Now, however, one of these genes is often found to be located on the bacterial chromosome. Two new plasmid-mediated dihydrofolate reductases have been identified. One of these enzymes (type IV) confers a low degree of insusceptibility but is inducible, to give levels 600 times higher than the bacterial host enzyme. The technique used to demonstrate the plasmids that carry the gene for this enzyme suggests that they would not be identified by the plasmid transfer methods in current use.