The biosorption equilibria and kinetics of an azo dye (reactive brilliant red K-2BP) were examined in this study using inactive carboxylmethylcellulose (CMC) immobilized Aspergillus fumigatus beads as the biosorbent. It was found that the biosorption capacity was at maximum when dye solution pH was about 2.0, that the sorption was spontaneous and endothermic with insignificant entropy changes, and that the Freundlich isotherm model fitted well to the biosorption equilibrium data. The biosorption rates were found to be consistent with a pseudo-second-order model. An intraparticle diffusion-based Weber-Morris model was applied to evaluate rate-limiting steps of the biosorption processes. The results suggested that the diffusion controlled the overall biosorption process, but the boundary layer diffusion of dye molecules could not be neglected. External mass transfer coefficients (beta(I)S) obtained by both Mathews and Weber model and Frusawa and Smith model were consistent.