Abstract The capability of a low-cost probiotic product was evaluated in Penaeus monodon reared under an outdoor culture system for a period of 90 days. The low-cost probiotic product was formulated by mixing the broth culture of Bacillus cereus along with commercial chalk powder in 1:1 ratio. The prepared probiotic product as a powdered supplement was incorporated along with the shrimp diet at various concentrations (1 to 5%). The viability of B. cereus in the experimental diet was tested once in 30 days up to 90 days during the experimentation. After 90 days of feeding experiments, the growth performance of shrimp was determined with a maximum production of 11.98 ± 0.367 g, SGR of 5.030 ± 0.171% and FCE of 86.63 ± 1.300% in P. monodon fed D4 diet (4% probiotic supplement). The immunological parameters determined at the end of the culture experiment showed variation among diets. The total haemocyte count (273.66 ± 3.09 x 105 cells/ml), phenoloxidase activity (0.132 ± 0.007 OD) and respiratory burst activity (0.291 ± 0.020 O.D) were high in P. monodon fed D4 diet. Similar effects were observed for plasma protein concentration (68.00 ± 1.41 mg/ ml), lysozyme activity (0.483 ± 0.014 U/ml) and bactericidal activity (81.0 ± 1.33%) in shrimp fed D4 diet. The results of the present investigation conclude that the probiotic product at a concentration of 4% was effective in elevating the growth and immune performances in shrimp P. monodon.