The efficient and complete conversion of lignocellulosic hydrolysates to ethanol as a result of fermentation processes makes possible the production of a bio-fuel competitive with petroleum. Traditional organisms used in ethanol fermentation, e.g. Saccharomyces cerevisise and Zymomonas mobilis, are unable to ferment pentose sugars. To overcome this problem genetic modifications of S. cerevisiae, Z. mobilis and E. coli were performed in order to produce ethanol directly from xylose and arabinose. The performance of the recombinant strains is encouraging: especially over the last 3 years the progress has been rapid. There is a possibility that strains of these three species may find a separate application in the fermentation of specific plant biomasses.