The behaviour of Pichia stipitis, Pachysolen tannophilus, Candida shehatae and Candida parapsilosis was investigated to select the most suitable yeast to convert xylose either to ethanol or to xylitol, with little or no formation of by-products. The aeration rate was used as a variable parameter. P. stipitis and C. parapsilosis were the most effective producers of ethanol and xylitol, respectively, both reaching productivities at very low levels of oxygenation. With P. stipitis, better ethanol productivity was attained under microaerobic conditions (K(L)a=4.8 h(-1)) while with C. parapsilosis high yields and rates of xylitol production were detected at K(L)a values of about 16.3 h(-1). P. tannophilus and C. shehatae showed lower performances under all conditions used while changes in oxygenation modified the ratio of ethanol to xylitol produced by these yeasts, suggesting that they are more dependent on the oxygen power input than P. stipitis and C. parapsilosis. The influence of oxygen transfer rates on ethanol and xylitol formation with the best producers is discussed.