Abstract: A growth medium is provided for mutant mixtures of Pichia stipitis or Candida shehatae that permits the growth of mutants which are the best xylose-to-ethanol fermenters in the mixture while inhibiting the growth of inefficient fermenters. The medium comprises a mixture of two compounds, the first compound selected from the group consisting of L-xylose, L-arabinose, D-arabinose, glycerol, erythritol, erythrose, 5- and 6-carbon polyols such as xylitol, L-arabinitol, D-arabinitol and mannitol, and wherein the second compound is ammonium tartarate or an inorganic nitrogen source compound such as ammonium chloride or ammonium sulfate.