Sundarbans biosphere reserve is a unique ecosystem and characteristic of diverse types of habitats ranging from mudflat to sandy beaches. Nutritionally, economically and ecologically important estuarine edible crab, Scylla serrata (Arthropoda: Crustacea: Decapoda) occupy diverse habitats and their survival is dependent on multidimensional immune responses mostly offered by hemocytes - the chief immunoeffector cells of hemolymph. The circulating hemocyte number is a stress indicator and hemocyte counts may be a valuable tool in monitoring the health status in biounsafe environment in S. serrata. The enumeration of total and differential hemocyte has been enumerated under the exposure of 1, 2 and 3 ppm of sodium arsenite for 1, 2, 3, 4, 7, 15 and 30 days in controlled laboratory condition. In S. serrata diverse subpopulations of hemocytes were identified as hyalinocytes, semigranulocytes and granulocytes. Alteration in hemocyte counts and density of hemocytes were recorded under 1, 2 and 3 ppm of arsenic in static water environment. Shift in THCs and DHCs recovered partially in arsenic exposed crabs maintained in arsenic free water up to 30 days. Exposure to 1 ppm of sodium arsenite for 30 days yielded a highest THC as 312.9 ± 3.6 x 105 cells / ml against control value of 47.31 9 ± 4.8 x 105 cells / ml. Relatively high THC in the crab maintained in toxin free water for 30 days of span indicates a persistence of cellular stress induced by arsenic exposure. A highest depression in hyalinocyte density was recorded under the exposure of 1 ppm of arsenite for 30 days as 1.17% against control value as 7.47%. Similarly decrease of granulocyte was recorded as 16.21% in 30 days of exposure against control value of 27.24%. Elevation of semigranulocyte was recorded as 82.62% against the control value of 65.29%. Study of possible restoration of DHC was carried out by maintaining the animals in toxin free water for 30 days. All groups of treated animals excepting the groups exposed to 2 ppm for 30 days and 3 ppm for 30 days recovered the DHC of the control. Depression in hyalinocyte and semigranulocyte density was recorded under the exposure of 3 ppm of arsenite for 30 days against control value. Arsenic induced alteration in hemocyte density indicated a state of immunological stress in estuarine crab. Data is suggestive of cellular stress which may lead to decline in endemic population of S. serrata in arsenic affected districts of West Bengal. © 2011 Nova Science Publishers, Inc.