Plasmepsin 4 from the malarial parasite Plasmodium malariae (PmPM4) is a member of the plasmepsins (Plasmodium pepsins), a subfamily of the pepsin-like aspartic proteases whose ortholog in the malarial parasite P. falciparum is involved in hemoglobin digestion in its digestive vacuole. Crystals of PmPM4 in complex with the small-molecule inhibitor AG1776 have been grown from a precipitant of 15% PEG 4000 and 200 mM ammonium sulfate in 100 mM sodium acetate pH 4.5. X-ray diffraction data were collected on a Rigaku rotating-anode generator from a single crystal under cryoconditions, with a maximal useful diffraction pattern to 3.3 A resolution. The crystals are shown to be orthorhombic and have been assigned to space group P2(1)2(1)2, with unit-cell parameters a = 95.88, b = 112.58, c = 90.40 A and a scaling Rsym of 0.104 for 14,334 unique reflections. Packing consideration and self-rotation function results indicate that there are two molecules per asymmetric unit. It is expected that in the near future the structure of PmPM4 will be obtained using molecular-replacement methods, obtaining phases from previously determined plasmepsin structures. Elucidation of the structure of PmPM4 in complex with inhibitors may be paramount to producing new antimalarial therapeutic agents.