Abstract. The bioavailability and toxicity of dissolved metals are closely linked to the metals' chemical speciation in solution. Normally the complexation of a metal by a ligand would be expected to decrease its bioavailability. The aqueous speciation of uranium undergoes tremendous changes within a pH range of 4 to 8 and in the presence of ligands commonly found in natural waters (carbonate, phosphate, hydroxide and natural organic matter). In the present project, we intend to establish links between speciation, medium composition and bioavailability of uranium. To study these links, we have chosen a unicellular green alga. Short-term metal uptake rates are determined in simple inorganic media with particular emphasis on the differentiation between adsorbed and intracellular metal. The Free-Ion Model (FIM) is tested to determine if U uptake is governed by the free uranyl species (U02 2 *) or if other species can be assimilated (i.e. passive diffusion of neutral, presumably lipophilic, species such as U0 2 (OH) 2 ; facilitated diffusion of metal bound to an assimilable ligand such as uranium-phosphate complexes). Results of our preliminary experiments indicate that complexation by phosphate decreases uranium bioavailability, as predicted by the FIM. However, uptake was stimulated when pH was increased from 5 to 7 in spite of the substantial decrease (55 -> 0.02 %) in free uranyl ion concentration in solution.