Batch biosorption experiments were carried out for the removal of malachite green a cationic dye from its aqueous solution using raw and thermally activated Pithophora sp., a fresh water algae as biosorbent. The operating variables studied are initial malachite green concentration, biomass concentration and solution pH. Pithophora sp. activated at 300 degrees C for 50 min posses a maximum sorption capacity for the range of initial dye concentrations studied (20-100 mg/L). The sorption kinetics were analysed using reversible first order kinetics, pseudo-first order, pseudo-second order model and the sorption data tend to fit very well in pseudo-second order model for the entire sorption time. The average pseudo-second order rate constant, K and initial sorption rate h were determined to be 3.46 x 10(-3) and 7.97 x 10(2) mg/g h. Equilibrium data are very well represented by Redlich Peterson isotherm model followed by Freundlich and Langmuir isotherm model. The maximum sorption capacity was 117.647 mg/g at 30 degrees C. The negative value of free energy change (-8.585 kJ/mol) indicates the spontaneous nature of adsorption. (c) 2005 Elsevier Ltd. All rights reserved.