Loss due to diseases is a major problem in crabs to control diseases, a number of antibiotics, sanitisers and chemicals are being used. This results in adverse environmental effects, emergence of antibiotic resistance and persistence of chemical residues in animal tissues. Considering the negative impact of antibiotics and other chemotherapeutants, biological control of pathogens would be a very useful strategy to prevent diseases in aquaculture and in this context, we looked into the potential for biocontrol of the crabs pathogen, The crab, O. senex senex was injected with A. hydrophila (0.1 ml of 109 cfu/ml). After injection of bacteria the crabs were allowed to withstand for 96 hrs. After 96 hrs one group of crabs was dissected, some tissues and haemolymph were used for biochemical assays. Remaining bacterial injected crabs were treated with 0.05 ml of 80% P. guajava leaves ethanol extract (1000 ppm), after 96 hours Micro protein, Total protein, glucose and Total cholesterol significantly increased in the experimental group. These result suggested that the P. guajava could combat the microbial infection by stimulating the immune response in crabs