Polychlorinated biphenyls (PCBs) and chlorinated pesticides were determined in several organisms and in the sediment sampled in the final stretch of the River Po (Italy). Bioconcentration models were used in order to test the equilibrium condition between biotic and abiotic compartments. The oligochaetes-sediment model fitted very well with the experimental results. The only exception was hexachlorobenzene. The results regarding the fish species demonstrate that most PCB congeners and p,p'-DDE are biomagnificated in the River Po ecosystem but to a lesser extent than expected on the basis of a four-step food chain model. In particular, most species approached to the third trophic level, while Perca fluviatilis concentrations correspond to a higher level but accumulate less than predicted by the model for a top predator.