ABSTRACT The objective of this study was to determine the digestible tryptophan requirement of tambaqui (Colossoma macropomum) fingerlings and its relationship with digestible lysine. A total of 300 tambaqui fingerlings with three initial weights (2.12 ± 1.19, 8.13 ± 0.75, and 15.18 ± 1.91 g) were distributed in a randomized complete block design, consisting of six treatments (0.225, 0.256, 0.288, 0.319, 0.350, and 0.381% of digestible tryptophan) and five replicates of 10 fish per plot. The tryptophan to lysine ratio was estimated using a 1.78% level of digestible lysine. Variables regarding performance, food efficiency, and body deposition of protein, fat, and ashes, and nitrogen retention efficiency were evaluated. Feed intake and nitrogen retention efficiency did not vary. For the digestible tryptophan intake, feed conversion and body deposition of ashes, the model that best fit was the Linear Response Plateau, with these variables improving with the elevation of digestible tryptophan concentration up to the levels of 0.335, 0.276 and 0.259%, respectively. Weight gain, specific growth rate, digestible tryptophan efficiency for weight gain, and body deposition of protein and fat increased in a quadratic manner with elevated digestible tryptophan levels; the estimated optimal levels were 0.320, 0.310, 0.280, 0.323 and 0.299%, respectively. The recommended digestible tryptophan level in rations for tambaqui fingerlings is 0.323%, corresponding to 0.108%/Mcal of digestible energy with a digestible tryptophan to lysine ratio of 18%.