Cytochrome c peroxidase [EC 1.11.1.5] was found in anaerobically grown yeast cells as well as aerobically grown cells. The enzyme was mainly recovered in a large particulate fraction from the anaerobic yeast cells where no mitochondrion is contained. The particles containing cytochrome c peroxidase seem to be vesicular, since the enzyme was readily extractable with hypotonic treatment. Membranes of the vesicular particles were so fragile that some portion of the enzyme was recovered in the supernatant fraction. Possible occurrence of peroxisomes in yeast cells was excluded, since catalase [EC 1.11.1.5] was mainly recovered in the supernatant fraction. When cytochrome c was produced by aeration of the anaerobic cells it was invariably found in the particles together with cytochrome c peroxidase irrespective of the degree of maturation of mitochondria. From these results it was suggested that, although no mitochondrion nor mitochondrial cytochrome is found in anaerobic yeast cells, the particles containing cytochrome c peroxidase are related to mitochondria, since the enzyme is localized in mitochondria in aerobic yeast cells.