The aim of this study was to evaluate dietary dosages of Lyngbya hieronymusii on the immune response and disease resistance against infections due to the opportunistic pathogen Aeromonas hydrophilia, in fish fingerlings of Platycephalus gibbosus. A cyanobacterium, L. hieronymusii was incorporated into the diets of P. gibbosus fingerlings and different biochemical, haematological and immunological parameters were evaluated. Superoxide anion production, lysozyme, serum bactericidal, serum protein and albumin were improved in cyanobacteria treated groups compared with the control group. Survival was increased in the cyanobacteria treatment group up to 89% survivability in the 500 mg cyanobacteria/kg and 1 g cyanobacteria/kg, and 75% survivability in the 2 g cyanobacteria/kg, respectively. These results indicate that L. hieronymusii stimulates the immunity and makes P. gibbosus more resistant to infection by A. hydrophilia.