Neurospora crassais capable of synthesizing two different alcohol dehydrogenases. The synthesis of each depends upon the carbon source on which the mycelium is grown. The fermentative alcohol dehydrogenase, consisting of one electrophoretic protein band, is produced when the mycelium is grown on sucrose. The oxidative alcohol dehydrogenase, consisting of at least two isozymes, is synthesized whenNeurospora crassais grown on ethanol as a sole source of carbon. This latter enzyme is repressed by sugars such as glucose or sucrose. The two enzymes have been differentiated (1) electrophoretically, (2) by their substrate specificity, (3) by the ratio of the forward and reverse reactions, and (4) by their thermostability. Extracts from acetate-grown cells indicate a mixture of the two enzymes.