The Melle-Boinot process (cell-recycle batch fermentation) was applied to the fermentation of xylose by two strains of the yeast Pachysolen tannophilus for periods of 31 days. This resulted in a significant increase in the ethanol yield and productivity from xylose by P. tannophilus NCYC 614 but not by strain CBS 4045. Analysis of cells of strain NCYC 614 at the end of the process revealed an increased ability to tolerate exogenous ethanol, a characteristic that was lost in the absence of selection pressure. It is postulated that the increased yield of ethanol from xylose by P. tannophilus NCYC 614 is a product of enhanced ethanol tolerance together with increased efficiency of the fermentation process associated with the Melle-Boinot system.