The emerging pollutants in diverse habitats have created a need for basic research towards profiling the structural and functional parameters ranging from cell to organs in a diversity of species, thus enabling realistic analyses of the risks imposed by the environmental stressors. In the present study, the circulating haemocytes and digestive gland of an edible bivalve mollusc from eastern India, Lamellidens marginalis, were investigated for morphological and functional attributes under the challenge of inorganic arsenite-an up-coming threat to the natural freshwater reserves of the Indo-Gangetic flood plains. The molluscs were exposed to three sublethal concentrations of sodium arsenite under controlled laboratory conditions for a maximum time span of thirty days. The toxic exposure caused significant alteration in the haemocytometric profile. It inhibited the activities of phosphatases, transaminases and acetylcholinesterase which are iconic for assessment of the physiological homoeostasis in the haemocytes and digestive tissue. At both cellular and tissue level, immune surveillance was compromised through inhibited generation of nitric oxide, phenoloxidase and superoxide anions. Moreover, exposure to sodium arsenite promoted xenometabolic and oxidative stress in both haemocytes and digestive gland by reducing the activity of glutathione S-transferase and catalase. It inflicted inflammatory damage and promoted neplasia in the digestive tissue as evident from the histopathological observations. The findings would be crucial to gauge the impending threats from inorganic arsenite exposure to the freshwater invertebrates. Further, it creates an avenue to speculate a new model for arsenic biomonitoring. © 2013 Elsevier Inc.