Tambaqui farming is expanding in Brazil and new technologies need to be implemented in order to achieve production increasing, more effective and sustainable, from the economic and environmental points of view. The aim of the present study was to evaluate the effect of feed supplementation with a commercial probiotic (Lacto-Sacc®), on juvenile tambaqui (Colossoma macropomum), at two stocking densities, regarding the following parameters: survival, growth (final weight, final length, weight gain), biomass production (relative biomass gain), feed use (apparent feed conversion, hepatosomatic index, and viscerosomatic index) and hematological parameters (hematocrit, hemoglobin concentration, erythrocyte count, mean corpuscular volume, mean corpuscular hemoglobin, mean corpuscular hemoglobin concentration and leukocytes respiratory burst activity). Tambaqui (n= 180, approximately 50 g) were distributed into 12 tanks (250 L), at two densities (2.5 kg m-³ and 5 kg m-³), in an entirely randomized design, with three repetitions. For each density, half of the stocked fish received feed supplemented with probiotics for 30 days, while the other half received non-supplemented feed. Increasing the stocking density did not influence fish survival (P > 0.05), although it caused reductions in growth and feed use in the groups stocked at the highest density. Changing the stocking density and adding probiotic supplementation to the tambaqui’s diet did not affected their hematological parameters. Probiotic supplementation caused fish growth when fish were stocked at the lowest density, probably due to better utilization of dietary nutrients
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