SUMMARYGrowth and respiration ofMyrothecium verrucariawere studied under aerobic and anaerobic conditions. The enzymes of the major respiratory pathways were assayed and the biochemical functions of the mitochondria examined. The fungus did not grow or produce carbon dioxide anaerobically. Almost all enzymes of the Embden-Meyerhof-Parnas pathway, the hexose-monophosphate shunt and the tricarboxylic-acid cycle were active in cell-free preparations. The exceptions were alcohol dehydrogenase and pyruvate decarboxylase. Ethanol was not produced by the mycelium under aerobic or anaerobic conditions, but lactic acid was formed. The mitochondria oxidized NADH and succinate, but not pyruvate. Oxidative phosphorylation occurred, but the P/O ratios were only 0.7 to 0.9.