Contracaecum rudolphii is the parasitic nematode of fish-eating birds. In the extracts from female, male and larvae L3 and L4 isolated from the alimentary tracts of black cormorants the activity of five antioxidant enzymes: superoxide dismutase (SOD), glutathione peroxidase (GPX), glutathione transferase (GST), glutathione reductase (GR), catalase (CAT) and the content of ascorbate and total antioxidative status (TAS) were determined. They can be put in order according to the activity growth: GPX, SOD, GST, CAT and GR. The activity of GPX were very low in the nematodes' extracts (1.23-7.67 microU/mg). CAT had higher activity (0.47-0.72 U/mg). The activity of GR was the highest (50.51-69.88 U/mg). SOD activity in the female was higher by ca. 50% than in the male while GST activity was at similar levels. GR and CAT activities were higher by ca. 30% in the male than in the female nematodes. GST and GPX activity and TAS in larvae L3 were significantly lower than in the adult nematodes or in L4 larvae. The activity of GPX, GR and CAT was lower in L4 larvae than in the adult male (p<0.05). The content of ascorbate was almost the same in all stages of parasite development (0.21-0.38 mg/g). The above results indicate differences in antioxidant systems related to both the sex and the developmental stage of C. rudolphii.