The magnetic properties of single crystals of SmCo5have been studied. The spontaneous magnetization was 894 emu/cm3at room temperature. The process of magnetization reversal was extremely sensitive to the surface condition. The magnetization reversal process for spheres with rough surfaces was dominated by free wall motion, while the magnetization for spheres with the smoothest surfaces reversed by means of a jump from the completely magnetized state to the completely reversed state. These latter spheres had an energy product equal to the limiting value of MGOe. Some preliminary data for NdCo5and YCo5are also presented.