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Two main probiotics (Lactobacillus plantarum and L. delbrueckii ssp. bulguricus) were isolated from the intestine of Barbus grypus and their characteristics were determined using biochemical, molecular and probiotical tests (bacterial antagonistic effects and lack of pathogenicity to fish). The effect that these lactic acid bacteria (LAB) had on the immune response of juveniles B. grypus and their resistance to Aeromonas hydrophila when fed as a dietary supplement was examined in the present study. The fish were randomly divided into four groups of 120 fish and were fed with diet containing 5 9 10 7 CFU g -1 of each isolated LAB, Lactobacillus casei PTCC 1608 or a control diet with no bacteria for 60 days, after which half of the fish from each group were challenged with A. hydrophila, while the remaining fish were fed with the control diets for a further 15 days. Various immune parameters were examined at 30 and 60 days post-feeding (dpf), and also 15 days after stopping feeding the probiotic. The results showed that dietary administration of L. plantarum significantly increased serum lysozyme, complement and respiratory burst activities in the serum of B. grypus by 30 dpf. The highest bactericidal activity was observed in the serum of fish fed the diet containing L. delbruecki ssp. bulguricus. Fish fed with diet containing L. delbruecki and L. plantarum had significantly higher survival rates than those fed the control diet after challenge with A. hydrophila. The results suggest that dietary supplementation with L. plantarum and L. delbrueck, isolated from B. grypus, can stimulate selected immune parameters in B. grypus infected with A.