Anaerobic xylulose fermentation was compared in strains of Zygosaccharomyces and Saccharomyces cerevisiae, mutants and wild-type strains to identify host-strain background and genetic modifications beneficial to xylose fermentation. Overexpression of the gene (XKS1) for the pentose phosphate pathway (PPP) enzyme xylulokinase (XK) increased the ethanol yield by almost 85% and resulted in ethanol yields [0.61 C-mmol (C-mmol consumed xylulose)(-1)] that were close to the theoretical yield [0.67 C-mmol (C-mmol consumed xylulose)(-1)]. Likewise, deletion of gluconate 6-phosphate dehydrogenase (gnd1 Delta) in the PPP and deletion of trehalose 6-phosphate synthase (tps1 Delta) together with trehalose 6-phosphate phosphatase (tps2 Delta increased the ethanol yield by 30% and 20%, respectively. Strains deleted in the promoter of the phosphoglucose isomerase gene (PGI1) - resulting in reduced enzyme activities - increased the ethanol yield by 15%. Deletion of ribulose 5-phosphate (rpe1 Delta) in the PPP abolished ethanol formation completely. Among non-transformed and parental strains S. cerevisiae ENY. WA-IA exhibited the highest ethanol yield, 0.47 C-mmol (C-mmol consumed xylulose)(-1). Other nontransformed strains produced mainly arabinitol or xylitol from xylulose under anaerobic conditions. Contrary to previous reports S. cerevisiae T23D and CBS 8066 were not isogenic with respect to pentose metabolism. Whereas, CBS 8066 has been reported to have a high ethanol yield on xylulose, 0.46 C-mmol (C-mmol consumed xylulose)(-1) (Yu et al. 1995), T23D only formed ethanol with a yield of 0.24 C-mmol (C-mmol consumed xylulose)(-1). Strains producing arabinitol did not produce xylitol and vice versa. However, overexpression of XKS1 shifted polyol formation from xylitol to arabinitol.