The effects of an acid-stable protease (RONOZYME ProAct) supplemented to a corn (Zea mays)-soybean (Glycine max) meal-based diet on apparent ileal digestibility (AID) of nutrients were evaluated in 120 weaned piglets (28 d old; 8.17 ± 0.90 kg). Pigs were divided into 2 equal groups and had free access to mash diet containing 0.4% Cr(2)O(3) as indigestible marker [basal diet (Std)] or this diet supplemented with RONOZYME ProAct at 15,000 PROT [the amount of enzyme that releases 1 μmol of pnitroaniline from 1 μM of substrate (Suc-Ala-Ala-Pro-Phe-p-nitroaniline) per min at pH 9.0 and 37°C)/kg (ProA). The ileal content was collected for the digestibility determination after euthanasia of 35 piglets of each group after 14 d of study and 25 piglets of each group after 29 d. Compared to group Std, AID of CP was increased (P < 0.05) after 29 d of treatment in group ProA. The AID of the indispensable AA, Met + Cys, and branched-chain AA was increased (P < 0.05) at the end of the study. In the protease supplemented pigs, the AID of the individual AA was not improved after 14 d of treatment whereas it was increased (P < 0.05) at the end of the experiment for Arg, Asp + Asn, Glu + Gln, His, Ile, Lys, Phe, Thr, Tyr ,and Val. In conclusion, dietary protease supplementation increased AID of AA in piglets.