A comparative study was carried out of superoxide dismutase (E.C.1.15.1.1) (SOD) and catalase activities in purified extracts of two trematodes: Fasciola hepatica and Dicrocoelium dendriticum. The superoxide dismutase activity was very similar to that measured in other eukaryotic cells. As no catalase activity was detected, the possibility that SOD in these trematodes might have the particular importance of removing superoxide radicals is discussed. The SOD isoenzymes of each species were analysed by polyacrylamide gel electrophoresis and the patterns revealed a quantitative difference in the number of isoenzyme bands: F. hepatica showed three, and D. dendriticum only two. Determinations were made of the in vitro inhibitory activities of four benzimidazoles and six synthesised pyrimidine derivatives on SOD from trematodes. The present results confirm that the percentage inhibitions of the pyrimidine derivatives are markedly superior to those produced by the benzimidazoles.