ABSTRACT:The study was conducted to determine the standardized ileal digestible (SID) sulfur amino acid (SAA) to lysine (Lys) ratio required to maximize the performance of 25-50 kg pigs fed reduced crude protein (CP) diets fortified with crystalline amino acids. A total of 360 crossbred (Duroc × (Landrace × Large White)) pigs, weighing 25.6 ± 2.7 kg, were blocked by gender, litter, and initial body weight (BW) and allotted to 1 of 5 dietary treatments with 6 pens per treatment and 12 pigs per pen for a 35-day performance trial. The basal diet was based on corn, soybean meal, and wheat bran and was formulated to be deficient in SAA (50% proportion of SID SAA to Lys). Graded levels of dl-methionine were added to the basal diet at the expense of wheat bran in order to provide 55.6, 60.0, 65.6, or 70.0% proportion of SID SAA to Lys, respectively. A constant SID Lys level of 0.90% was set so that Lys was the second limiting amino acid (AA) in all diets. Average daily gain (ADG) and feed conversion ratio (FCR) improved (linear and quadratic, P < 0.05) with increasing dietary proportion of SID SAA to Lys. Increasing the dietary proportion of SID SAA to Lys decreased the serum urea nitrogen (SUN) level (quadratic, P < 0.05). A two-slope broken-line model estimated the optimum proportion of SID SAA to Lys to be 62.2, 61.5, and 62.3% for maximum ADG and minimum FCR and SUN, respectively, whereas a curvilinear-plateau model yielded an optimum proportion of SID SAA to Lys level of 63.8, 62.5, and 61.5% for maximum ADG and minimum FCR and SUN, respectively. Based on an average of these estimates, we conclude that the proportion of SID SAA to Lys required for 25-50 kg pigs fed low CP diets is 62.3%. This estimate is higher than the NRC (2012) recommendation of 56.1% for 25-50 kg pigs fed normal CP diets. Keywords: amino acid ratio; requirementList of abbreviations: SID = standardized ileal digestible, SAA = sulfur amino acid, Lys = lysine, Met = methionine, Cys = cysteine, Trp = tryptophan, Val = valine, CP = crude protein, BW = body weight, AA = amino acid, ADG = average daily gain, FCR = feed conversion ratio, ADFI = average daily feed intake, SUN = serum urea nitrogen, NE = net energy