The effect of Mg on Pichia stipitis growth and ethanol production was studied under condition of constant oxygen uptake rate (OUR). Biomass/xylose and biomass/Mg+2 yields increased with Mg+2 concentration with a maximum value at Mg+2 4mM, ethanol being the main product obtained. At low Mg+2 levels (ImM) 49 % of carbon flux to ethanol was redirected to xylitol production, accomplished through NADH intracellular accumulation.