A large cyclotron is described with particular reference to magnet design, ``ion‐source'' characteristics, radiofrequency requirements, optimum shape of the magnetic field, and the distribution in range of the high energy alpha‐particle beam. Increasing the ``ion‐source'' electron current above 400 ma was found to lower the efficiency of production of high speed ions. Investigation of the radiofrequency power losses showed that the anodes of the oscillators absorbed 35 percent of the total power, while the tank coil across the ``dees'' and the ``dees'' themselves took, respectively, 30 percent and 21 percent. The optimum arrangement of iron shims used for magnetic field correction was such that the average field at the middle of the pole was decidedly higher than at the edge.