When the time of flight principle is applied to isotope separation in a magnetic field, a separation in time is obtained which depends directly upon the difference in isotopic masses. Modulation of the ion beam is necessary for collection of separated isotopes. The relative merits of intensity, direction, and energy modulation are discussed. Separation of uranium isotopes, with low enrichment, was achieved by energy modulation. The cyclorator is found to have the same fundamental limitations as the calutron, the most significant being loss of resolution with increased throughput.