Sundarbans biosphere reserve is an ecologically important area of West Bengal, which supports a wide variety of estuarine species including Scylla serrata estuarine mudcrab of economic value. In this present study, activity of phosphatases (acid and alkaline) of hemocytes was estimated under the sublethal exposure of arsenic in controlled static water environment for 1, 2, 3, 4, 7 and 15 days of exposure. Exposure to 3 ppm of sodium arsenite for 15 days, resulted a maximum inhibition of activity of acid phosphatase (ACP) as 0.016 microM mg(-1) protein min(-1) with intermediate activity in different spans of exposure against the control value of 1.83 microM mg(-1) protein min(-1). Similar mode of maximum inhibition of activity of alkaline phosphatase (ALP) was recorded as 0.008 microM mg(-1) protein min(-1), when the crabs were exposed to 3 ppm of sodium arsenite for 15 days against the control value of 0.694 microM mg(-1) protein min(-1). Exposure to 1 ppm of sodium arsenite caused inhibition of activity of ACP which ranged from 1.76 to 0.70 microM mg(-1) protein min(-1). For 2 and 3 ppm of exposure activity of ACP ranged from 1.12 to 0.42 and 0.67 to 0.016 microM mg(-1) protein min(-1) respectively. For the activity of ALP exposure to 2 and 3 ppm of sodium arsenite yielded an inhibition of activity which ranged from 0.472 to 0.279 and 0.275 to 0.008 microM mg(-1) protein min(-1) respectively. Arsenic exposure resulted a dose dependent decrease in the activities of both of the phosphatases and may be considered as a suitable biomarker of aquatic pollution in Sundarbans mangrove.