dietary dosages of Lyngbya majuscula on the immune response and disease resistance against infections due to the opportunistic pathogen Aeromonas hydrophilia, in fish fingerlings. Cyanobacteria, Lyngbya majuscula was incorporated into the diets of fish, Mugil cephalus (Mangrove snapper) fingerlings. Every 20 days, different biochemical, haematological and immunological parameters were analyzed. Superoxide anion production, lysozyme, serum bactericidal, serum protein and albumin were enhanced in cyanobacteria treated groups compared with the control group. After 60 days, fish were survival with Aeromonas hydrophilia, and mortality percentage was recorded up to day 10 post-challenge. Survival decreased in control group (57%) up to day 10 after infection. However, this was increased in the cyanobacteria treatment group, i.e. 85% survivability in the 1 g cyanobacteria/kg, 5 g cyanobacteria/kg and 71% survivability in the 10 g cyanobacteria/kg, respectively. These results indicate that Lyngbya majuscula stimulates the immunity and makes Mugill cephalus more resistant to infection by A. hydrophilia.