In waste streams containing hexose and pentose sugars, xylose is only converted when the glucose concentration is sufficiently low not to be inhibitory. In an attempt to ferment glucose and xylose simultaneously in a single reactor without the addition of oxygen, Pichia stipitis was immobilized in alginate beads. In the core of the beads, a low hexose concentration is expected, thus allowing xylose conversion. The conversion of hexose and pentose appears to be simultaneous. When P. stipitis was immobilized in alginate, a glucose conversion rate of 0.13 g g-1h-1 was found. Due to this low rate, the glucose concentration in the core of the beads was too high to allow xylose conversion. Independently of the initial cell concentration, the biomass concentration only doubled once during these experiments. Another possibility to convert both glucose and xylose is the coimmobilization of Saccharmoyces cerevisiae with P. stipitis to attain higher glucose conversion rates. With coimmobilized S. cerevisiae and P. stipitis, simultaneous conversion of glucose and xylose was possible. The rate of xylose conversion depended on the dilution rate in the reactor and the initial P. stipitis concentration in the beads. Cell counts in the beads showed that S. cerevisiae formed a dense layer at the periphery of the beads, but that P. stipitis hardly grew under these conditions. The poor growth of P. stipitis in alginate beads leads to the need for high initial cell concentrations to reach a more complete conversion of mixed substrates. © 1990.