The major form of glutathione S-transferase (GST) activity from the mosquito Anopheles dirus (species B), a vector of malaria in Thailand has been purified 421-fold. It constituted approx. 20% of the total measured CDNB conjugating activity in the homogenate. This enzyme appeared as a single band of 25.0 +/- 0.26 kDa on SDS-PAGE and was kinetically characterized with 10 substrates and 4 inhibitors. The enzyme is capable of catalysing dehydrochlorination of 1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane (DDT) in vitro at a rate of 4.4 nmol of 1,1-dichloro-2,2-bis-(p-chlorophenyl)ethane (DDE) formation per mg protein. This is comparable to the rate of catalysis of the orthologous isoenzyme from An. gambiae reported previously. The IC50 plots of the inhibitor data (fractional velocity vs log [I]) for three of the inhibitors indicate the homogenous nature of this enzyme. However, inhibition by ethacrynic acid demonstrates more than a single affinity site for interaction. The six N-terminal amino acids of the purified enzyme are identical to a GST reported from Aedes aegypti, which was indicated to play a role in DDT-resistance in this species. The results suggest that the two enzymes may belong to the same class, however each possesses a different specificity.