Robust microorganisms are necessary for economical bioethanol production. However, such organisms must be able to effectively ferment both hexose and pentose sugars present in cellulosic biomass to ethanol. Wild type S cerevisiae strains rapidly ferment hexose, but not pentose sugars.  Considerable efforts, including ours, were made to genetically engineer S. cerevisiae to ferment xylose. Our genetically engineered Saccharomyces yeast, 424A(LNH-ST), containing overexpressed xylose reductase, xylitol dehydrogenase, and xylulokinase, is able to ferment glucose and xylose as well as other hexose sugars to ethanol. In the present study, genes responsible for the conversion of xylose to xylulose with different co-factor requirements as well as genes in the pentose passway were overexpressed in our host cells.  To improve the balance of the cofactors, (i) fungal GDP1 enzyme from Kluyveromyces lactis, (ii) xylose reductase from N. crassa and C. parapsilosis, and (iii) xylitol dehydrogenase from P. stipitis containing 3 mutations, R276H/K270R/N272D, were overexpressed.  Four PPP genes, TKL1, TAL1, RKI1 and RPE1 from S. cerevisiae, were also over-expressed together in our strain 424A(LNH-ST).  The ethanol and the byproducts produced during xylose fermentation by these new recombinant strains were analyzed.  Overexpression of GDP1 lowered xylitol production by more than 50%. However, other strains carrying combinations of different xylose reductases and xylitol dehydrogenases as well as new strains containing the overexpressed PPP genes did not yield any significant improvements in xylose utilization. The reasons for the limited improvements in xylose utilization and ethanol production by these new recombinant yeast strains will be discussed.