Aspartic proteases play key roles in the biology of malaria parasites and human immunodeficiency virus type 1 (HIV-1). We tested the activity of seven HIV-1 protease inhibitors against cultured Plasmodium falciparum. All compounds inhibited the development of parasites at pharmacologically relevant concentrations. The most potent compound, lopinavir, was active against parasites (50% inhibitory concentration [IC50], 0.9 to 2.1 microM) at concentrations well below those achieved by ritonavir-boosted lopinavir therapy. Lopinavir also inhibited the P. falciparum aspartic protease plasmepsin II at a similar concentration (IC50, 2.7 microM). These findings suggest that use of HIV-1 protease inhibitors may offer clinically relevant antimalarial activity.