The main purpose of this work was to compare the performance of two different kinds of reactors (CSTR and CPFR) in order to enhance the ethanol productivity in the fermentation of D-xylose by Pichia stipitis immobilized in x-carrageenan. Immobilization was carried out in a 4% aqueous suspension of x-carrageenan, which was mixed with the inoculum. The bioparticles were treated with A1(NO3)3 as hardening agent. The fermenters operated during a long period of time (about 30 d). Best results were obtained in the packed-bed reactor (CPFR), which allowed operation at high final ethanol concentrations, this fact having been explained because of the observed strong product inhibition. The overall productivity reached values higher than 3.8 g/(L.h). This supposed an interesting improvement with relation to the productivities found in the literature, which as an average did not exceed 1 g/(Lh). However, the specific productivities of yeast in the continuous stirred tank reactor (CSTR) were always greater because the bioparticles were kept in close contact with the broth, whereas in the CPFR, there were at least two problems: (a) the possibility that the produced gas could prevent the intimate contact between the substrate and the particles and (b) the possible existence of preferential paths.