The aim of this study was to compare the in vitro and in vivo activities of superoxide dismutase, an enzyme acting against free-radicals, in Trichinella spiralis and T. pseudospiralis isolates. In vitro, the activity of the superoxide dismutase in T. pseudospiralis was determined to be about two times higher than in T. spiralis. Only one band representing superoxide dismutase was found by electrophoretic analysis. In vivo, in muscles infected with T. spiralis, a strong histochemical reaction in the capsule and the layer directly adhering to it was found. In muscles infected with T. pseudospiralis, the activity was most marked in the vicinity of the larva and becomes less marked with the distance. The character of the reaction indicated that the enzyme is excreted by the larvae.