The effects of tryptophan (Trp) supplementation were investigated in growing pigs (n = 100 from 40 to 100 kg) fed ad libitum in a context of a reduction of the crude protein (CP) level. A control diet with a CP content close to the standard level (NP; 16.5%) was compared to four low CP diets (LP; 13.7%) gradually supplemented with L-Trp (0, 0.3, 0.6 and 0.9 g/kg for LP0, LP1, LP2 and LP3, respectively). All the diets were formulated to contain similar net energy (9.9 MJ/kg) and standardized ileal digestible (SID) lysine (Lys; 0.81%) contents. The actual SID Trp:Lys ratios were 22, 17, 20, 23 and 27% for NP, LP0, LP1, LP2, and LP3 diets, respectively. Growth performance decreased when pigs were fed a low CP diet without L-Trp supplementation (LP0 vs. NP, -160 g/d; P < 0.05). The average daily gain of pigs fed the LP1 diets was similar to that obtained with NP treatment (958 vs. 989 g/d; P > 0.05). Similar growth performances were measured in LP1, LP2 and LP3 groups. In our experimental conditions, the SID Trp:Lys ratio in growing pigs fed a low CP diet has to be at least 20% to optimize the growth performance.