Studies have shown that fungal organisms possess the capacity of heavy metal removal. In the present investigation, a study in batch system was conducted using the macrofungus, Agaricus bisporus as biosorbent for evaluating its potentiality to adsorb lead (II) ionsfrom aqueous solution. To improve the bioadsorption capacity, the biosorbent was pretreated with NaOH, Na2CO3 and NaHCO3 . Pretreatment with NaOH resulted in a significant improvement in bioadsorption capacity. The experimental equilibrium data were adjusted by adsorption isotherms from Langmuir and Freundlich models and their equilibrium parameters were determined. Forthe untreated biosobent, the Langmuirmodelsupplied qmax = 28.8 mg/g and b = 0.02 L/mg and for the Freundlich model parameters are Kf = 1.7 and n =1.8. For the sodium hydroxide (NaOH) treated biomass, the parameters were qmax = 34.6 mg/g, b= 0.08 L/mg, Kf = 5.3 and n = 2.3. The best adjusted model to the experimental equilibrium data for both untreated and sodium hydroxide (NaOH) treated biosorbent was the Langmuir model.