Three allyl-alcohol-resistant mutants were isolated in the dimorphic fungus Mucor rouxii and characterized with regard to their alcohol dehydrogenase (ADH) activity in vitro and in vivo as well as their ability to execute the morphological alternatives of dimorphism under different environmental stimuli, either in the absence or in the presence of oxygen. These studies indicated that fermentation and yeast-cell development are independent events and that ADH activity is essential for growth of the fungus in the absence of oxygen. Heterokaryon construction and analysis indicated that in the three mutant strains the corresponding genetic alterations are recessive nuclear mutations which behave as allelic in complementation tests.