Glutathione transferase (GST) activities toward the selective herbicide fenoxaprop-ethyl, together with thiol contents, have been compared in seedlings of wheat (T riticum aestivum) and two populations of black-grass (Alopecurus myosuroides) which are resistant to a range of herbicides (Peldon and Lincs E1), and a black-grass population which is susceptible to herbicides (Rothamsted). GST activities toward the non-cereal herbicides metolachlor and Ñuorodifen were also determined. On the basis of enzyme speciÐc activity, GST activities toward fenoxaprop-ethyl in the leaves were in the order wheat [ Peldon \ Lincs E1 [ Rothamsted, while with Ñuorodifen and metolachlor the order was Peldon \ Lincs E1 [ Rothamsted [ wheat. Using an antibody raised to the major GST from wheat, which is composed of 25-kDa subunits, it was shown that the enhanced GST activities in both Peldon and Lincs E1 correlated with an increased expression of a 25-kDa polypeptide and the appearance of novel 27-kDa and 28-kDa polypeptides. Leaves of both wheat and black-grass contained glutathione and hydroxymethylglutathione, with the concentrations of glutathione being in the order Peldon [ Lincs E1 \ Rothamsted \ wheat. However, in glasshouse dose-response assays, the Lincs E1 population showed much greater resistance to fenoxaprop-ethyl than Peldon. We conclude that high GST activities and the availability of glutathione may contribute partially to the relative tolerance of black-grass to herbicides detoxiÐed by glutathione conjugation. Although herbicide-resistant populations show enhanced GST expression, in the case of fenoxaprop-ethyl the associated increased detoxifying activities alone cannot explain the di †erences between populations in the degree of resistance seen at the whole plant level.