Methionine is an essential amino acid, and generally, the first limiting one in the diets of tambaqui, the main native fish produced in continental aquaculture in South America. However, there is a lack of information on their amino acid requirement, especially for maintenance and efficiency of utilization. The present study aimed to determine the requirement of [methionine plus cystine] for the maintenance and efficiency of utilization using tambaqui of different body weights. Two experiments

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... ere carried out using 250 tambaqui with weights of 100 and 300 g. They were distributed in a completely randomized design with four treatment groups (0.264, 0.396, 0.528, and 0.660% of digestible [Met + Cys]), five replications. The control treatment (CT) was performed with the addition of methionine at the first level (0.264%) until the second level was reached (0.396%). The maintenance requirement was determined by the regression analysis between the consumption of [Met + Cys] and protein retention equal to zero. The efficiency was determined using the value of the slope coefficient of the line obtained by the linear regression. The protein and methionine retention values of tambaqui weighing 100 to 300 g were compared by the parallelism test, and no difference was observed between the parameters of the equations indicating the need for only one equation to describe the responses to body weights. The requirement of digestible [Met + Cys] for the maintenance of tambaqui with weights from 100 to 300 g was 60.47 mg kg-0.7 day-1 and the efficiency of utilization of 42%.