A catalase-negative mutant of the yeast Hansenula polymorpha consumed methanol in the presence of glucose when the organism was grown in carbon-limited chemostat cultures. The organism was apparently able to decompose the H 2 O 2 generated in the oxidation of methanol by alcohol oxidase. Not only H 2 O 2 generated intracellularly but also H 2 O 2 added extracellularly was effectively destroyed by the catalase-negative mutant. From the rate of H 2 O 2 consumption during growth in chemostat cultures on mixtures of glucose and H 2 O 2 , it appeared that the mutant was capable of decomposing H 2 O 2 at a rate as high as 8 mmol · g of cells −1 · h −1 . Glutathione peroxidase (EC 1.11.1.9) was absent under all growth conditions. However, cytochrome c peroxidase (CCP; EC 1.11.1.5) increased to very high levels in cells which decomposed H 2 O 2 . When wild-type H. polymorpha was grown on mixtures of glucose and methanol, the CCP level was independent of the rate of methanol utilization, whereas the level of catalase increased with increasing amounts of methanol in the substrate feed. Also, the wild type decomposed H 2 O 2 at a high rate when cells were grown on mixtures of glucose and H 2 O 2 . In this case, an increase of both CCP and catalase was observed. When Saccharomyces cerevisiae was grown on mixtures of glucose and H 2 O 2 , the level of catalase remained low, but CCP increased with increasing rates of H 2 O 2 utilization. From these observations and an analysis of cell yields under the various conditions, two conclusions can be drawn. (i) CCP is a key enzyme of H 2 O 2 detoxification in yeasts. (ii) Catalase can effectively compete with mitochondrial CCP for hydrogen peroxide only if hydrogen peroxide is generated at the site where catalase is located, namely in the peroxisomes.