The statistical properties of Pc 1 magnetic pulsations at sunchronous orbit have been examined by using measurements obtained with the UCLA flux-gate magnetometer on board ATS 1. In 1967, 121 intervals of Pc 1 activity occurred. These events typically had amplutudes of 1-7 &ggr; and periods of 2-5 s. Most events lasted less than 30 min and occurred in the afternoor sector. Pc 1 waves were more probable during the main phase of a geomagnetic storm, within 1 hour of a substorm expansion. While they do not have the signature of IPDP (intervals of pulsations of diminishing periods) events at ATS, simultaneous ground records show the occurrence of rising frequency tones characteristic of IPDP events in seven out of ten cases. The wave frequency ranged principally from 0.1 to 0.2 of the local proton gyrofrequency, similar to observations of electron cyclotrons waves. The observed Pc 1 activity is consistent with the generation by cyclotron resonance of 30-keV protons in a 30.cm3 plasma.