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This study investigated the effects of various standardized ileal digestible(SID)Trp to Lys ratios on the performance and carcass characteristics of late finishing gilts receiving low CP(9.6%)diets supplemented with crystalline AA.Ninety gilts(89.1 ± 5.1 kg)were utilized in a dose-response study conducted for 35 days.Crystalline Trp(0,0.1,0.2,0.4 or 0.6 g/kg)was added to a corn-wheat bran basal diet providing SID Trp to Lys ratios of 0.12,0.15,0.18,0.21 or 0.24.Each diet was fed to 6 pens of pigs with 3 gilts per pen.At the end of experiment,30 gilts(one pig per pen)were slaughtered to evaluate carcass traits and meat quality(BW = 121 kg).Increasing the SID Trp to Lys ratio increased ADG(linear and quadratic effect,P <0.05)and also improved G:F(linear and quadratic effect,P <0.05).Serum urea nitrogen(SUN)decreased as the SID Trp to Lys ratio increased(linear and quadratic effects,P <0.05).A quadratic effect of L*light and marbling in the Longissimus dorsi was observed as the dietary SID Trp to Lys ratio increased(P <0.05).Increasing the SID Trp to Lys ratio increased the level of serum GH(quadratic effect,P <0.05)and also increased the level of serum insulin-like growth factor(IGF-1)(linear andquadratic effect,P <0.05).Increasing the SID Trp to Lys ratio increased the protein abundance of the muscular amino acid transporter of sodium-coupled neutral amino acid transporter 2(SNAT2)in the Longissimus dorsi muscle(linear and quadratic effect,P <0.05).The optimum SID Trp to Lys ratios to maximize ADG and G:F as well as to minimize SUN levels were 0.16,0.17 and 0.16 using a linear-break point model and 0.20,0.20 and 0.20 using a quadratic mode.l Tryptophan may regulate performance by altering the levels of muscular amino acid SNAT2 transporters through the up-regulation of hormones such as GH and IGF-1.