SUMMARYA high incidence of resistance to trimethoprim has been shown in the normal faecal flora in a population in south India. The dihydrofolate reductase (dhfr) genes mediating transferable resistance to trimethoprim have been identified.Unusually, in this study, the dhfrV was shown to be the predominant resistance gene (dhfrV 50 % of transconjugants, dhfrla 30%), the dhfrlb was also detected being distinguished from the dhfrV by an oligo-probe. However, when nontransferable resistance was considered, the dhfrIa was the most prevalent of the dhfrs identified. All those plasmids harbouring the dhfrla were shown to possess Tn7. All the plasmids that probed positive for the dhfrV and the dhfrlb were shown to be associated with the integrase of the Tn2l-like transposons, but 8 of the dhfrV genes were not associated with the Tn2l resolvase. The dhfrIV was shown to be present in all seven plasmids that produced low level trimethoprimresistance. The dhfrV, first characterized in Sri Lanka, would seem to have a local distribution in this region of Asia but is distinguishable from the dhfrlb only by the use of an oligo-probe.