Mitochondrial malic enzyme from the parasitic roundworm Ascaris suum has been isolated and crystallized. The enzyme functions as a tetramer with 605 amino acids per monomer, catalyzing the oxidative decarboxylation of malate to pyruvate and carbon dioxide using NAD as the oxidizing cofactor. The enzyme crystallizes at pH 7.5 from polyethylene glycol 4000 containing magnesium sulfate, NAD, and tartronate. The crystals are oï the trigonal space group P3<1>21 or the enantiomorph with a=b=130.7(5)A and c=152.9(4)A. Native data to 2.5A were collected at the SSRL synchrotron. Self-rotation function analysis using GLRF (Rossman) shows 222 molecular symmetry with one two-fold axis coincident with a crystallographic two-fold. Molecular replacement studies with AMORE (J. Navaza, CCP4) and using a structurally conserved portion of a bacterial lactate dehydrogenase monomer (PDB entry 1LLD) gave two significant peaks approximately 180 degrees apart. The results of molecular replacement and heavy atom searches are presented.