Individual PCBs and chlorinated pesticides (CHLPs) were determined in water, suspended particles (SPM), sediments, and Asiatic clams to study their bioaccumulation kinetics and evaluate the usefulness of the clams as sentinel organisms. Bioaccumulation depended on the routes of uptake and bioavailability determined by compound hydrophobicity. CHLPs were mainly present in the dissolved phase, showed a positive correlation between the bivalve-water BAFs and Kow's and reached steady-state levels before 70 days in a caging experience. These facts reflect an efficient water-gill partitioning process. In contrast, PCBs were only detected in the SPM, showed essentiaIIy Kow, independent bivaIve-sediment and bivalve-SPM BAFs with the lowest values for superhydrophobic heptachlorobiphenyls 174,180, and 170, and did not reach steady-state levels during a 140-day exposure period. This suggests that the uptake was hindered by their stronger affinity for SPM and the interference of steric factors during intestine absorption. Corbicula fluminea are reliable sentinel organisms and showed clear geographical trends along 150 km of the Rlo de La Plata coast: a progressive decrease of PCB levels and more degraded patterns dominated by recalcitrant congeners, i.e., 153, were observed with increasing distance from the major La PIata-Buenos Aires urban center. A size-related trend overlapped with this spatial gradient: older clams usually showed higher levels and a more degraded PCB signature. CHLPs showed less clear geographical trends due to the presence of multiple sources. CHLP patterns showed a strong dominance of chlordan e - re la ted com po u n ds, pa rti c u I a rly transchlordane which was 2-5 times more abundant than the cis isomer. On a temporal scale, from 1986 to 1993, Asiatic clams presented sustained PCB levels but showed a significant decrease of chlordane and DDT, which suggests reduced inputs in recent years.Facultad de Ciencias Naturales y Museo