92RESTORATION OF LACTATE DEHYDROGENASE LEVELS BY PSIDIUM GUAJAVA LEAF EXTRACT IN A VIBRIO HARVEYI INFECTED FRESHWATER CRAB OZIOTELPHUSA SENEX SENEXMortality due to diseases is a major problem in culturing crabs. To control diseases, a number of antibiotics, sanitizers 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, control of pathogens by phytochemicals would be a very useful strategy to prevent diseases in aquaculture and in this context, we looked into the potential for control of the crab pathogens. The crab, O. senex senex was injected with V. harveyi ( 0.1 ml of 10 8 cfu/ml) and were allowed to withstand for 96 hrs. After 96hrs a group of crabs were dissected, some tissues and haemolymph were used for bio chemical assays, remaining bacterial injected crabs were treated with 0.05 ml of 80% P. guajava leaf ethanol extract (1000 ppm), after 96 hours Lactate Dehydrogenase (LDH) level significantly decreased in the experimental group. The results suggest that the P. guajava leaf extract could combat the microbial infection by stimulating the immune response and also by reducing the increased LDH level due to V. harveyi infection in crabs.nathanvimal18@yahoo.comResearch522015LDH,V. harveyi,O. senex senex,P. guajava,Fresh water crabsBernard Mark Asirvatham A,Sekhar PP.G. and Research Department of Zoology, Voorhees College, Vellore - 632001, Tamil Nadu, India,P.G. and Research Department of Zoology, Voorhees College, Vellore - 632001, Tamil Nadu, India