Macrobrachium amazonicum is the native freshwater prawn species with the greatest potential for captive production in Brazil. The stable isotope carbon and nitrogen technique (δ 13 C and δ 15 N) is efficient to determine the contribution of different food sources to a given organism. We used this technique to estimate the contribution of the food sources in the biomass gain of M. amazonicum. Fish fillets were used to determine the isotopic fractionation between juveniles of M. amazonicum and a standard food source. Juvenile M. amazonicum were submitted to four treatments with different food availability to determine their contributions; T1: without soil substrate and with feed supply; T2: without soil substrate and feed supply; T3: with soil substrate, feed supply and addition of an aquatic macrophyte (Elodea sp.); and, T4: with soil substrate, aquatic macrophytes and without feed supply. Periphyton, plankton and precipitated organic material were present in all treatments. The isotopic fractionation was 0.57 ± 0.07 (carbon) and 2.14 ± 0.18 (nitrogen). The prawn fed with ration presented growth 2.4 and 2.82 times higher in the treatments without substrate (T1) and with substrate and macrophytes (T3), respectively. The contribution of the food sources reinforces omnivore in prawn. The benthic organisms and feed supply were fundamental for the better growth performance of M. amazonicum. Estimating the effective contributions of food sources can help in developing diets more adequate for the species, increasing productivity, reducing costs and reducing the environmental impact of waste substances.