Hemoproteins occuring in anaerobically grown cells of Saccharomyces cercuisiae were studied spectrophotomctrically. The absorption band around 590 mμ, previously attributed to cytochromc a1, might be rather due to cytochrorae c peroxidase [EG 1. 11.1.5], The reduced minus oxidized difference spectra of intact cells also indicated the occurrence of a pigment, cytochrome b1, which was similar in spectral properties and redox behavior to cytochrome b5 of liver microsomes. The hemoprotein responsible for the 450 mμ maximum in the CO difference spectrum was found to be almost identical in many respects with P-450 of liver microsomes. Semi-anaerobically grown cells were very rich but acrobically grown cells were very poor in P-450 content. The absorption peak around 420 mμ in the CO spectrum of anaerobic cells was a composite band arising from the presence of CO-binding components, cytochrome c peroxidase and an unidentified hemoprotein. In aerobic cells, yeast hemoglobin and catalase [EC 1.11.1.6] were also included in the peak in addition to the two hemoproteins. Both yeast P-450 and cytochrome b1 were recovered in paniculate fractions upon cell fractionation. Essentially the same hemoprotein pattern was observed in cells of a respiration-deficient yeast strain.