The biosorption of Methylene Blue, a cationic dye, onto dried Rhizopus arrhizus, a filamentous fungus, was examined in the absence and in the presence of increasing concentrations of sodium dodecylsulfate (SDS), an anionic surfactant. The fungus exhibited the maximum dye uptake at an initial pH value of 10 in the absence of surfactant. The addition of SDS did not change the initial pH of maximum dye uptake. Dye uptake by the fungus increased with increasing initial dye concentration up to 1100 mg l(-1). The presence of 1 mM surfactant in biosorption medium enhanced the dye removal dramatically. The Freundlich model better described the equilibrium dye uptake than the Langmuir model. According to the Langmuir model, the maximum dye uptake was determined as 370.3 mg dye g(-1) of dried biomass in the absence of surfactant. When 1 mM (288.4 mg l(-1))SDS was added to the biosorption medium, this value raised to 1666.6 mg g(-1) resulting in 4.5-fold increase in uptake capacity. The pseudo-second-order kinetic model described the biosorption kinetics accurately for all cases studied confirming that a chemisorption process controls the sorption rate. (C) 2010 Elsevier B.V. All rights reserved.