The hyperbenthic fauna of the Ria Formosa, a tidal lagoon in southern Portugal, was sampled with a hand-pushed sledge during the winter 1997. The hyperbenthic community in the lagoon differed from that of the adjacent sandy beaches of the Atlantic Ocean, as well as from that of the main river discharging into the lagoon. Calculations of density, biomass and diversity revealed higher values for the community of the lagoon. The salinity of the Ria Formosa doesn't differ from the open sea but there is a difference in exposition: the beach along the Atlantic Ocean is more exposed to currents than the beaches inside the lagoon and that is probably the main reason why a different community is found in the open sea. A different community was also found in the upstream part of the river which is correlated with the reduced salinity due to freshwater input.