Three hundred and ninety six reverted Nile tilapia(Oreochromis niloticus), from Thailand strain, averaging initial weightof 0.86 ± 0.02 g were used to evaluate the effect of feeding methionineplus cystine:lysine levels, based on the ideal protein concept, with deficientlevels of digestible lysine (1.40%). The experiment was analyzed as a completelyrandomized design, with six treatments, six replications by treatment and 11fishes for experimental unit. The treatments consisted of five diets with differentratios of digestible methionine + cystine:lysine (59.5, 63.5, 67.5, 71.5 and75.5%) and digestible threonine:lysine ratio of 80.0%; and one diet with thedigestible methionine plus cystine:lysine ratio of 75.5% and threonine: digestiblelysine ratio of 85.0%, all isoenitrogenous and isoprotein. The fishes were fedad libitum six daily meals during 40 days. Performance parameters, bodycomposition, body protein and fat deposition and nitrogen retention efficiencywere evaluated. The increasing dietary digestible methionine plus cystine:lysineratio did not affect the studied parameters, except for the body fat and humiditycontent, that showed quadratic effect, with quadratic maximum and minimum at67.0 and 66.7%, respectively. However, it was observed treatment effect (75.5%)on feed:gain ratio, nitrogen retention efficiency and protein and lysine efficiencyfor growth. It was concluded that the total or digestible dietary sulfur aminoacid level of 0.91 and 0.86%, that correspond to a total and digestible methionineplus cystine:lysine ratio of 57.2 and 59.5%, respectively, provided the bestresults of performance and carcass characteristics of Nile tilapia fingerlings.
