Clavispora opuntiae cells were immobilized in Na-alginate beads to produce ethanol from D-xylose, using a packed-bed column reactor. Immobilized gel beads were treated with 0.075 M Al(NO3)(3) solution for 10 min as hardening agents. Maximum ethanol yield (Y-P/S, 0.462 +/- 0.003 g(p) g(s)(-1)) (i.e., 90.60 +/- 0.07%) of the theoretical) was obtained at a dilution rate of 0.09 h(-1). The maximum volumetric and specific rates for ethanol production (Q(p) and q(p)) and sugar uptake (Q(s) q(s)) were found to be (3.10 +/- 0.01 g(p) l(-1) h(-1) and 0.21 +/- 0.01 g(p) g(x)(-1) h(-1)) and (8.06 +/- 0.02 g(s) l(-1) h(-1) and 0.54 +/- 0.02 g(s) g(x)(-1) h(-1)), respectively, at a dilution rate of 0.31 h(-1) without any loss in the immobilized cell reactor, which was operated over a period of 45 days at a dilution rate of 0.24 h(-1) without any loss in the immobilized cell activity. Beyond this period, a slight decrease in the immobilized cell activity was observed. In this type of reactor, there were at least two problems: (i) the possibility that the produced gas could prevent the intimate contact between the substrate and the immobilized gel beads and (ii) the possible existence of preferential paths.