The presence of NADH-cytochrome b5 reductase [EC 1.6.2.2] in microsomes from anaerobically grown yeast was confirmed by its isolation and purification. The purified preparation of the reductase showed an apparent molecular weight of 27,000 daltons. The reductase appeared to contain loosely-bound FAD as a prosthetic group. The reductase required NADH as a specific electron donor, and could reduce some redox dyes as well as cytochrom b5. The reductase, however, could not reduce cytochrome c. Michaelis constants of the reductase for NADH and calf liver cytochrome b5 were 6.3 and 1.5 micron M, respectively, and optimal pH for cytochrome b5 reduction was 5.6. Although some differences exist between the properties of NADH-cytochrome b5 reductase from yeast and from mammalia, the results indicate a functional similarity of the present enzyme to mammalian NADH-cytochrome b5 reductase in the microsomal electron-transport system.