Sundarbans Biosphere Reserve of India supports a wide range of flora and fauna. Ecologically, this reserve is a mangrove forest with numerous estuarine creeks. Mudcrab Scylla serrata (Crustacea: Decapoda) is an important member of estuarine habitat of Sundarbans. Indiscriminate collection of juvenile crab seeds and contamination of estuarine water pose a serious threat for this aquatic animal. Generally invertebrates including mudcrab lack adaptive immune system and depend on innate immune defense to combat pathogen and toxin . Cellular immune response involved the recruitment of immunocompetent hemocytes capable of performing adhesion, aggregation, phagocytosis, generation of intracellular cytotoxic molecules etc. In this present study, spectrum of innate immunological reactivity was examined in juvenile mudcrab under the exposure of arsenic, a toxic metalloid of aquatic environment. Experimental juveniles were exposed to 1, 2 and 3 ppm of sodium arsenite for 1, 2, 3 and 4 days in controlled laboratory condition. Total hemocyte count is a stress indicator of pollution which was enumerated both in the control and arsenic exposed S. serrata. Crabs exposed to sodium arsenite expressed an alteration in total hemocyte density. Aggregation and adhesion of hemocytes are metabolic behaviours whereas phagocytosis is a classical immunological response of hemocytes and arsenic exposure yields a dose responsive decrease in aggregation, adhesion and phagocytosis against the control indicating perturbation of blood cell hemeostasis, alteration of hemocyte surface characteristics and immune suppression. Capability of generation of cytotoxic molecules like superoxide anion and nitric oxide are reported as a potential strategy of immunological defense against pathogen invasion. Disruption of hemocyte density, adhesion, aggregation, phagocytosis and increment of cytotoxic response may lead to a state of alteration of immunity. Arsenic induced alteration of immune status may impart a state of vulnerability in juvenile crab inhabiting the toxic aquatic environment. Such a situation may reduce the ecological fitness of mudcrab to combat against the toxic challenge caused by arsenic pollution. © 2012 Nova Science Publishers, Inc.