To better understand the relationship between lead speciation and bioavailability in natural freshwaters, the interaction of lead with the freshwater alga Chlorella kesslerii was studied in the presence of the Suwannee River fulvic acid (SRFA). Special attention was paid to direct interactions of the fulvic acid on the algae, as well as potential physiological (membrane permeability and algal metabolism) influences. Lead-free ion concentration measurements were carried out using a novel ion-selective electrode. Pb uptake decreased in the presence of SRFA with respect to noncomplexed Pb, but uptake fluxes, cellular Pb, Pb bound to the transport sites, and total adsorbed Pb were all higher than predicted from Pb2+ activities, in accordance with the free ion activity model (FIAM). The discrepancies between the observed values and those predicted by the FIAM in the presence and absence of synthetic ligands increased with increasing concentration of SRFA. Several hypotheses were examined to explain the observed differences. No contributions of labile and/ or hydrophobic Pb-SRFA complexes were found. Furthermore, direct biological effects, including variations in membrane permeability or algal metabolism, could not account for the observations. On the other hand, changes in the algal surface charge due to SRFA adsorption seemed to account, at least partially, for the observed increase in lead uptake in the presence of SRFA as compared to that corresponding to the same Pb2+ concentration in the presence of synthetic ligands.