The nickel biosorption capacity of a Streptomyces rimosus biomass treated with NaOH (0.1 M) was studied in the batch mode. After pre-treatment of biomass at the ambient temperature, optimum conditions of biosorption were found to be: a biomass particle size between 50 and 160 pm, an average saturation contact time of 2 h, a biomass concentration of 3 g L-1 and a stirring speed of 250 rpm. The external mass transfer was found to be the controlling step in the overall sorption process. The equilibrium data could be fitted by Langmuir isotherm equation. Under these optimal conditions a biosorption capacity of 32.6 mg Ni2+ /g biomass was obtained. (C) 2004 Elsevier Ltd. All rights reserved.