BACKGROUND: The bioconversion of whey into ethanol by immobilized Kluyveromyces marxianus in packed-bed and fluidized bioreactors is described. Both batch and continuous cultures were analyzed using three different strains of K. marxianus and the effect of the operating mode, temperature, and dilution rates (D) were investigated. RESULTS: All immobilized strains of K. marxianus (CBS 6556, CCT 4086, and CCT 2653) produced similar high yields of ethanol (0.44 +/- 0.01 g EtOH g-1 sugar). Significant variations of conversion efficiencies (66.1 to 83.3%) and ethanol productivities (0.78 to 0.96 g L-1 h-1) were observed in the experiments with strain K. marxianus CBS 6556 at different temperatures. High yields of ethanol were obtained in fluidized and packed-bed bioreactors continuous cultures at different D (0.1 to 0.3 h-1), with the highest productivity (3.5 g L-1 h-1) observed for D = 0.3 h-1 in the fluidized bioreactor (87% of the maximal theoretical conversion), whereas the highest ethanol concentration in the streaming effluent (28 g L-1) was obtained for D = 0.1 h-1. Electronic micrographs of the gel beads showed efficient cell immobilization. CONCLUSION: Batch and continuous cultivations of immobilized K. marxianus in fluidized and packed-bed bioreactors enable high yields and productivities of ethanol from whey. Copyright (C) 2012 Society of Chemical Industry