During a 3-hour interval on August 21 and 22, 1978, a compressional Pc 5 event was observed by the ISEE-1 magnetometer and medium energetic particle experiment instrument. During the event the ISEE spacecraft was inbound near the equator and near the dusk meridian at distances between 11 and 7 Re. The finite Larmor radii of the energetic protons allow us to determine unambiguously both the azimuthal and radial components of the phase velocity, and consequently the wave length and the azimuthal wave number. A 2n&pgr; ambiguity in evaluating the phase velocity is removed by finding a consistent and physically reasonable solution for different energy channels. As the spacecraft approached the plasmapause, a quasi-sinusoidal wave form having a 165-s period was observed; it was found to be propagating azimuthally westward with a phase velocity of about 33 km/s and an azimuthal wave number of 60. In the outer magnetosphere, more irregular wave forms were observed. These waves also propagated azimuthally westward and may possibly have had a small earthward (radial) component of phase velocity. To examine the effect of heavy ions on the estimated wavelength, we have derived the perturbation of differential flux by calculating the first-order perturbed phase space distribution of plasma consisting of two ion species. We found that, for the detector we used, the effect is very small even there is a significant fraction of heavy ions present. This is because for each energy channel, the detector response to the heavy ions is more sensitive to those with higher energy than those with the nominal proton energy, while the flux drops exponentially with the energy. In all cases the modulation amplitude of ion fluxes decreases with an increasing ratio of the ion Larmor radius to the wavelength. ¿ American Geophysical Union 1988