Respiratory and fermentative pathways coexist to support growth and product formation in Pichia stipitis. This yeast grows rapidly without ethanol production under fully aerobic conditions, and it ferments glucose or xylose under oxygen-limited conditions, but it stops growing within one generation under anaerobic conditions. Expression of Saccharomyces cerevisiae URA1 (ScURA1) in P. stipitis enabled rapid anaerobic growth in minimal defined medium containing glucose when essential lipids were present. ScURA1 encodes a dihydroorotate dehydrogenase that uses fumarate as an alternative electron acceptor to confer anaerobic growth. Initial P. stipitis transformants grew and produced 32 g/l ethanol from 78 g/l glucose. Cells produced even more ethanol faster following two anaerobic serial subcultures. Control strains without ScURA1 were incapable of growing anaerobically and showed only limited fermentation. P. stipitis cells bearing ScURA1 were viable in anaerobic xylose medium for long periods, and supplemental glucose allowed cell growth, but xylose alone could not support anaerobic growth even after serial anaerobic subculture on glucose. These data imply that P. stipitis can grow anaerobically using metabolic energy generated through fermentation but that it exhibits fundamental differences in cofactor selection and electron transport with glucose and xylose metabolism. This is the first report of genetic engineering to enable anaerobic growth of a eukaryote.