Abstract We have measured and characterized three oxidant defense enzymes in early and late intraerythrocytic stages of the human malarial parasite, Plasmodium falciparum. Isolated early intraerythrocytic stages contain catalase (24.1 μmol min −1 (mg protein) −1) and superoxide dismutase (SOD; 6.3 units (mg protein) −1) but little or no glutathione peroxidase (GPX; <2 μmol min −1 (mg protein) −1). Isolated late intraerythocytic stages of P. falciparum contain slightly less catalase (17.0 μmol min −1 (mg protein) −1) but significantly more GPX (7.7 μmol min −1 (mg protein) −1) and SOD (25.1 units (mg protein) −1). P. falciparum, like P. berghei, probably acquires most of its SOD from its host, since parasite-associated SOD is predominantly cyanide-sensitive, and has the same p I as host SOD. Unlike P. berghei, however, late stages of P. falciparum contain an additional SOD isozyme which is not cyanide-sensitive and may represent an endogenous enzyme. Parasites grown in red cells that have been partially depleted of SOD are more sensitive to exogenously generated superoxide, suggesting some dependence of the parasite on host SOD.