Ascaris suum NAD-malic enzyme catalyzes the decarboxylation of oxalacetate and reduction of pyruvate. Thus, the present classification (E.C. 1.1.1.39) for this enzyme should be changed to E.C. 1.1.1.38. In the absence of nucleotide, both the chicken liver NADP-malic enzyme and Ascaris suum NAD-malic enzymes catalyze the decarboxylation of oxalacetate. A study of the pH dependence of kinetic parameters for oxalacetate decarboxylation and pyruvate reduction was carried out for the NAD(P)-malic enzyme with Mg^2+ and Mn^2+ in the presence and absence of nucleotide. In all cases, an enzyme residue is required in its protonated form for reaction while for oxalacetate decarboxylation the β-carboxyl of oxalacetate is required unprotonated. Of a number of inhibitory binding analogs of malate tested, oxalate is the tightest binding inhibitor for Ascaris suum enzyme.