In this study, biosorption of C. I. Acid Red 18 from aqueous solution was investigated by using acid treated biomass of Penicilium sp. in a batch system. The effects of initial pH, contact time, initial dye concentration and temperature on dye biosorption were investigated. Optimum pH for efficient dye biosorption was found to be 3.0 for acid treated biomass. The bosorption capacity was increased with the increasing temperature in studied temperature range. The Langmuir and Freundlich isotherm models were applied to experimental equilibrium data and the Langmuir model better described the equilibrium dye uptake than the Freundlich model. Thermodynamic studies revealed that the biosorption process was successful, spontaneous and endothermic in nature.