This study addresses removal of a basic dye, methylene blue, from aqueous Solutions by using dried Ulothrix sp. biomass as biosorbent. The effects of the initial dye concentration, contact time. temperature, solution equilibrium pH, biosorbent dosage, and mixing rate on biosorption of the dye have been investigated. It was found that 30 min is sufficient in order to reach adsorption equilibrium. The amount of methylene blue adsorbed onto Ulothrix sp. increased with increasing equilibrium pH and mixing rate, in contrary, it decreased with increasing temperature and sorbent dosage. The process followed the pseudo-second-order kinetic model. The isosteric enthalpy and entropy values were calculated as -11.8 kJ/mol and 37.5 J/(mol K). respectively. In addition, the results suggest that the physical interactions between sorbent particles and sorbate ions play an important role for the adsorption of methylene blue onto the biosorbent. (C) 2009 Elsevier B.V. All rights reserved.