The large Argentine marine littoral zone is characterized by great number of wetlands and includes only one coastal lagoon, Mar Chiquita, which has been declared as a Biosphere Reserve by the Man and Biosphere Reserve Program from UNESCO. Its margins present populations of Chasmagnathus granulatus, a semiterrestrial crab distributed along wide salinity gradients that plays an important role as a key species within the corresponding trophic web. Dissolved cadmium (Cd) and zinc (Zn) concentrations present in this ecosystem were determined. Cadmium concentrations ranged between n.d. and 0.82 mug/L and zinc levels ranged between n.d. and 1224.38 mug/L within the mentioned coastal lagoon. Cd and Zn acute semistatic toxicity bioassays were carried out for 96 h on juvenile crabs of C. granulatus. LC(50) 96-h values were 2.24 mg Cd(2+)/L and 7.07 mg Zn(2+)/L at 5 psu, and 15.42 mg Cd(2+)/L and 11.41 mg Zn(2+)/L at 25 psu. Higher salinities resulted in lower metal toxicity. This effect was stronger for Cd than for Zn. C. granulatus juveniles LC(50) 96-h values determined for Cd were three to four orders of magnitude higher than the corresponding dissolved metal concentrations in the Mar Chiquita coastal lagoon; nevertheless, those Zn values determined were similar to several ones corresponding to natural water samples.