Mercury bioaccumulation kinetics of two important macrobenthic species, the polychaete Hediste diversicolor and the bivalve Scrobicularia plana, were evaluated following a dietary pathway (i.e. contaminated algae), through a mesocosm laboratory experiment. Both studied species pre-sented a similar model of Hg bioaccumulation kinetics, a linear pattern of accumulation through time being the mercury accumulation in the organ-isms proportional to the mercury concentration in the food. Mercury bioaccumulation rates were higher in the polychaete H. diversicolor (reaching approxi-mately 0.15 μg g −1 at the end of the experiment) than in the bivalve S. plana (≈0.07 μg g −1), which could be related to their feeding strategies, ingestion rates and assimilation efficiencies. Moreover, the mercury bioaccumulation revealed to be quite a fast process especially for the polychaete, and despite the fact that this species is not an edible organism, it is an important prey item, which can greatly contribute to the transport of contaminants to higher trophic levels. Therefore, the bioaccumulation of mercury by these important macrobenthic species, especially the bivalves, represents a non-negligible risk for humans.