Glutathione S-transferases (GSTs) from Fasciola hepatica have been purified by glutathione affinity chromatography. Two closely migrating species of Mr 26,000 and 26,500 were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and several species resolved by two-dimensional gel analysis, indicating substantial heterogeneity among the GSTs. N-terminal amino acid sequencing revealed one core sequence containing three polymorphisms, whereas the sequence of GST peptides implied a minimum of three different GSTs. The amino acid sequence data assigned the F. hepatica GSTs to the mu class of GSTs with high similarities to these proteins in other helminths and mammals. The native GSTs of F. hepatica appeared to behave as dimers as determined by molecular sieving chromatography. The observation that the GSTs of F. hepatica are heterogeneous in sequence and behave as dimers in the native state suggest that these isoenzymes may exhibit considerable functional heterogeneity which may be of importance to the parasite. Immunocytochemical studies suggest that the main source of GST in F. hepatica are the parenchymal cells and peripheral tissues of the parasite. Some extracellular GST is associated with the lamellae of the intestinal epithelium. The identification of an intestinal GST is unique among trematodes studied to date.