| Summary: | The Bahía Blanca Estuary includes a large tidal plain with an area close to 1150 km<sup>2</sup>. Mud is predominant in its sediments, where a significant population of the crab <i>Chasmagnathus granulata</i> lives during the whole year. Moreover, there are important urban and industrial discharges into this environment. Cd and Pb concentrations were determined in samples of water (for both dissolved and suspended particulate matter) and surface sediments (total and 63 µm fractions). Organic matter was analysed in the sediments, while temperature, salinity, pH and dissolved oxygen were measured in the estuarine water. The metal concentrations determined in this study were: 1.47 ± 1.08 µg Cd g<sup>–1</sup> and 14.68 ± 4.31 µg Pb g<sup>–1</sup> in surface sediments; 2.21 ± 1.33 µg Cd g<sup>–1</sup> and 25.70 ± 7.09 µg Pb g<sup>–1</sup> in the <63 µm sediment fraction; 0.18 ± 0.11 µg Cd g<sup>–1</sup> and below the analytical detection limit of Pb in suspended particulate matter. Furthermore, dissolved Cd and Pb were lower than the norms for marine and estuarine waters (2 µg dm<sup>–3</sup> for Cd and 5 µg dm<sup>–3</sup> for Pb, E.P.A.). <br> Simultaneously, the effects of Cd and Pb were studied on recently hatched larvae of <i>Ch. granulata</i>, through 96-hour semi-static acute assays. Viability was the criterion assessed in the assays. LC<sub>50</sub> (96 h) for Cd was 46.43 µg dm<sup>–3</sup> (36.92 - 56.34 µg dm<sup>–3</sup>, whilst that for Pb was 1.00 mg m<sup>–3</sup> (0.79 - 1.23 mg dm<sup>–3</sup>, which demonstrates that Cd is more toxic towards larvae. Finally, both LC<sub>50</sub> values determined for Cd and Pb were higher than the corresponding metal concentrations measured in the Bahía Blanca environment.
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