The general concern for arsenic in the marine environment is associated with its wide distribution and potential toxicity. In the present work, concentrations and chemical speciation of arsenic were characterized in sediments and representative biota from the Indian Sundarban, the largest continuous mangrove tract formed at the mouth of the Hugli (Ganges) River estuary, northeastern part of the Bay of Bengal. Analyzed organisms included both shellfish (Macoma birmanica, Sanguinolaria acuminata and Meretrix meretrix) and finfish (Liza parsia, Liza tade, Harpodon nehereus and Eleutheronema tetradactylum). Arsenic concentrations in sediments did not exceed 4 μg g-1 dry weight with the contribution of inorganic molecules (arsenate and arsenite) ranging from 61.7 to 81.3%. Total As (TAs) concentrations varied from less than 2 to 16 μg g-1 in tissues of bivalves; the more elevated As accumulation was observed in gills and the mantle probably due to ion exchange properties of the mucous layer covering these organs, whereas adductor muscle and the podium showed limited values. Distribution of various arsenic compounds followed a quite similar trend in bivalve tissues; arsenobetaine (AsB) was the most dominant form followed by compounds such as dimethylarsinate (DMA), trimethyl arsine-oxide (TMAO), tetramethyl arsonium (TETRA) and arsenocholine (AsC), while inorganic arsenic (IAs) represented a minor constituent (0.2 to 6.9%). Among the fish, detritivorous/herbivorous species (L. parsia and L. tade) exhibited T As concentrations of 10.8 and 9.71 μg g-1 dry wt with a prevalence of AsB (52-67%) and TETRA (26-35%); higher concentrations of T As were measured in the two carnivorous species (20.62 and 19.67 μg g-1 dry wt, in H. nehereus and E. tetradactylum respectively) mostly as AsB (63.3-72.3%) and AsC (17.5-28.6%). The obtained results can be considered as baseline levels for arsenic in the investigated area, confirming the predominance of organoarsenicals in marine organisms compared to more toxic inorganic compounds. Considering the ecological importance of this ecosystem and the increasing anthropogenic impact, the distribution of arsenic through the food chain should be continuously monitored, using organisms of different feeding guilds as indicators. © 2013 The Royal Society of Chemistry.