The production of bioethanol from lignocellulose hydrolysates requires a D-xylose-fermenting microorganism. Recombinant Saccharomyces cerevisiae strains derived from CCTCC M94055 and CAT-1, both used for bioethanol production, were constructed by incorporation of genes involved on xylose metabolism. Genes selected for these constructions are required for two main pathways described for xylose metabolism. One group contains XR (xylose reductase) and XDH (xylitol dehydrogenase) genes from Scheffersomyces stipitis, modified for host optimization. In the other group, XI (xylose isomerase) gene from Piromyces sp. and from Streptomyces coelycolor with nucleotidic sequence optimized for yeasts are being assayed. Two different strategies were used for the modification of the yeasts. One strategy involved the use of two pairs of cassette genes organized in tandem and surrounded by two fragments of about 200nts identical to specific regions of GRE3 gene. These cassettes were synthetized and inserted by homologous recombination in both chromosomes. These strains exhibited reduced ability to grow on xylose. As alternative strategy, we are using plasmids encoding for antibiotic resistance, as vectors for the introduction of heterologous genes involved on xylose metabolism.