We have analyzed periodicities in the occurrence rate of proton flares for solar cycles 19 through 21 (1955-86) and have identified two epochs that exhibit a 154-day periodicity. These epochs are a 14-year interval from 1958 January through 1971 December and a 5.5-year interval from 1978 February to 1983 August. The best-determined period is 154.6. We have found that the phase of this periodicity changed between the above-mentioned two epochs by about one half of a period. It appears that the occurrence rate of proton flares is more sensitive to the 154-day periodicity than the occurrence rate of flares selected by other criteria. Keywords: Solar activity, Sunspots, Astrophysics, Periodic variations, Proton flares, Sun.