Bioassays of two sites along the Rio Negro in Uruguay indicate ecotoxicity, which could be attributable to trace concentrations of lead in river sediments. Monthly samples at two sites at Baygorria and Bonete locations were analyzed for both particle size and lead. Lead was determined by atomic spectrometry in river water and sediment and particle size by sieving and sedimentation. Data showed that Baygorria 0s sediments have greater percentage of clay than Bonete 0s (20.4 and 5.8%, respectively). Lead was measurable in Baygorria 0s sediments, meanwhile in Bonete 0s, it was always below the detection limit. In water samples, lead was below detection limit at both sites. Bioassays using sublethal growth and survival test with Hyalella curvispina amphipod, screening with bioluminescent bacteria Photobacterium leiognathi,and acute toxicity bioassay with Pimephales promelas fish indicated toxicity at Baygorria, with much less effect at Bonete. Even though no lethal effects could be demonstrated, higher sublethal toxicity was found in samples from Baygorria site, showing a possible concentration of the contaminant in the clay fraction.