A method is described for the isolation of phosphomannose isomerase from dried brewers' yeast. The isolated enzyme, after 1500- to 1600-fold purification, has a specific activity of approximately 800 µmoles of substrate converted per min per mg at 30°. It was found to be homogeneous by the criteria of sedimentation velocity and sedimentation equilibrium ultracentrifugation, ion exchange chromatography, gel filtration, and zone electrophoresis on cellulose acetate. Concomitant with the homogeneity measurements, the following physical parameters were established: s20, w (c = 5 mg per ml), 3.86 S; D20, w (boundary spreading during velocity sedimentation, c = 5 mg per ml), 7.78 × 10−7 cm2 sec−1; D20, w (gel filtration), 7.62 × 10−7 cm2 sec−1; molecular weight, either by velocity sedimentation and diffusion or by equilibrium sedimentation, 45,000 ± 1000.