Experiments were conducted to determine the tryptophan (Trp) requirement of 10- to 20-kg pigs. A Trp-deficient experimental diet (.11% total Trp, 18% CP, 3,320 kcal of ME/kg) was composed of corn, feather meal, corn gluten meal, soybean meal, and dried whey. True digestibility of Trp in the experimental diet was 70% (.077% digestible Trp) as established in a digestibility assay that involved cecectomized, adult cockerels (Exp. 1). An initial pig study (Exp. 2) verified that the experimental basal diet, when fortified with sufficient L-Trp, was capable of producing growth rate and feed efficiency similar to that of pigs fed a conventional corn-soybean meal-dried whey diet (18% CP, 3,320 kcal of ME/kg). In Exp. 3, crossbred pigs with an average BW of 10.9 kg were fed the Trp-deficient basal diet supplemented with 0, .015, .030, .045, .060, and .075% L-Trp. In Exp. 4, crossbred pigs that averaged 9.5 kg were fed the basal diet fortified with .030, .045, .060, and .075% L-Trp. By examining the data from Exp. 3 and 4 together, the digestible Trp requirement for maximum daily weight gain was estimated to be .14% of the diet. Assuming an 88% true digestibility of Trp in commercial diets based on corn and soybean meal (calculated from published data), the total Trp required in practice would be .16% (.89% of the dietary protein).