The biosorption of copper(II) and lead(II) from aqueous solutions on Aspergillus niger pretreated with NaOH was investigated to be a function of initial pH, initial metal ion concentration and biosorbent concentration. The optimum pH values were determined as 5.0 and 4.0 for copper(II) and lead(II), respectively. Biosorption capacity values of the biomass at these pH values increased with increasing initial metal ion concentration and the maximum capacities were obtained as 25.5 and 28.9 mg g^-1 at 250 mg litre^-1 initial copper(II) and lead(II) concentration, respectively. The Freundlich, Langmuir and Redlich-Peterson isotherm models were applied to the experimental equilibrium data. The model parameters were estimated by the linear regression for both the Freundlich and Langmuir models and non-linear regression for the Redlich-Peterson model. It can be seen that the equilibrium data fitted very well to the Langmuir adsorption model at all pH values studied.