In order to understand the variation of total mercury (THg) in a tropical mangrove lagoon ecosystem. THg concentration was seasonally analysed in the biota and surface sediments from Setiu lagoon, Terengganu,Malaysia. THg in sediments showed spatially the similar trend with higher concentrations in dry season compared to the rainy season, whereas most of the biota was found the opposite trend between seasons.Increased rainfall during the rainy season may be the main factor enhancing the bioavailability of THg to biota. Fishes are recorded the highest THg accumulation compared to other groups. However, the THgaccumulation in carnivorous fishes suggests the need for a long-term THg monitoring programme in the mangrove lagoon