The toxicity of cadmium and chromium to Pseudokirchneriella subcapitata and Microcystis aeruginosa was evaluated through algal growth rate during 96h exposure bioassays. Free metal ion concentrations were obtained using MINEQL super(+) 4.61 and used for IC50 determination. Metal accumulations by the microorganisms were determined and they were found to be dependent on the concentration of Cd super(2+) and Cr super(6+). IC50 for P. subcapitata were 0.60 mu mol L super(-1) free Cd super(2+) and 20 mu mol L super(-1) free Cr super(6+), while the IC50 values for M. aeruginosa were 0.01 mu mol L super(-1) Cd super(2+) and 11.07 mu mol L super(-1) Cr super(6+) times P. subcapitata accumulated higher metal concentrations (0.001 -0.05 mu mol Cd mg super(-1) dry wt. and 0.001 -0.04 mu mol Cr mg super(-1) dry wt) than the cyanobacteria (0.001 -0.01 mu mol Cd mg super(-1) dry wt and 0.001 -0.02 mu mol Cr mg super(-1) dry wt). Cadmium was more toxic than chromium to both the microorganisms.