A set of 61 relatively uncontaminated Pc 4-5 micropulsations from the interval July 1 throught October 10, 1974, were examined for longitudinal phase changes. The data were obtained from the east-west line of magnetometers operated by the University of Alberta over the summer and early fall of 1974. From our analysis a direct correlation between longitudisal phase shift and frequency in found which is of the form m= (1.4¿0.4)f+0.26, where f is the frequency in millihertz and m is the azimuthal wave number derived from the measured phase shift. This relationship indicates a phase velocity which is independent of frequency and whose magnitude is Vϕ?14 km/s. The sign of the phase shift, and thus the direction of apparent phase velocity, changes abruptly 1-2 hours before local noon. The signals appear to propagate away from this prenoon region toward the terminators; that is, they move westward in the morning hours and eastward during the afternoon hours. Both the magnitude and the sign of this phase velocity are consistent with proposed generation mechanisms which invoke the Kelvin-Helmholtz instability at the magnetopause boundary.