We have analysed the influence of the initial pH of the medium and the quantity of aeration provided during the batch fermentation of solutions of D-xylose by the yeast Hansenula polymorpha (34438 ATCC). The initial pH was altered between 3.5 and 6.5 whilst aeration varied between 0.0 and 0.3 vvm. The temperature was kept at 30 degrees C during all the experiments. Hansenula polymorpha is known to produce high quantities of xylitol and low quantities of ethanol. The most favourable conditions for the growth of xylitol turned out to be: an initial pH of between 4.5 and 5.5 and the aeration provided by the stirring vortex alone. Thus, at an initial pH of 5.5, the maximum specific production rate (mu(m)) was 0.41 h(-1), the overall biomass yield (Y-x/s(G)) was 0.12 g g(-1), the specific D-xylose-consumption rate (q(s)) was 0.075 g g(-1) h(-1) (for t = 75 h), the specific xylitol-production rate (q(Xy)) was 0.31 g g(-1) h(-1) (for t = 30 h) and the overall yields of ethanol (Y-E/s(G)) and xylitol (Y-Xy/s(G)) were 0.017 and 0.61 g g(-1) respectively. Both q(s) and q(Xy) decreased during the course of the experiments once the exponential growth phase had finished.