In aquatic ecosystems, the potential effects of toxic compounds are greatly determined by their physicochemical forms and bioavailability. The aim of this work was the assessment of metal toxicity to the green alga Pseudokirchneriella subcapitata in contaminated water samples of an urban river (Matanza-Riachuelo, Argentina). A total of 30 samples were analyzed. Physicochemical parameters and total and dissolved metal (Cd, Cu, Cr, Pb, Ni and Zn) concentrations were measured. Toxicity was related as a function of size fractionation and chemical speciation (using the program Mineql+) in the dissolved fraction (<1.2μm). Percentage inhibition of algal growth (%Iw) was estimated as the differences between cultures with and without EDTA additions in the filtered water samples. Concentrations of Cd (1-85μg.L-1), Cr (3-164μg.L-1), and Ni (10-99μg.L-1) in river waters were always lower than the respective EC50 for P. subcapitata, whereas concentrations of Cu (5-306μg.L-1), Pb (13-546μg.L-1), and Zn (4-380μg.L-1) in dissolved fractions resulted above the EC50 values in many samples. Only 10 samples showed algal growth inhibition (between 11.21% and 42.19%), which 8 of them also showed high concentrations of free ionic forms of Pd or Zn. Free ionic concentrations of Cu were always lower than the EC50 for the algae. Two samples resulted toxic to P. subcapitata in spite of the absence of toxic metal concentrations. Besides, 13 samples no inhibited the algal growth and high concentrations of Pd and Zn were obtained, producing expected %Iw values between 10.03% and 90.51%. The absence of toxicity in most samples with toxic metal concentrations could be related to the presence of organic ligands, colloidal organic matter, and antagonism effects among other metals, which reduce their bioavailability.