Ultra-low-frequency modulation of ion fluxes is frequently observed by the NOAA low-energy particle detector on the synchronous satellite ATS 6. This modulation is usually associated with simultaneous periodic variations in the magnetic field. During the first 8 months of operation, 14 ULF modulation events have been identified, all with the unusual property that maximum flux modulation occurs in the smallest pitch angle detector. This paper examines one of these events in detail to determine the properties of the flux and field variations and the phase relations between the magnetic field and the various energy channels of three differently oriented particle telescopes. For this event it is found that maximum flux modulation occurs in the 100- to 150-keV detectors at an angle of 32 ¿ to the ambient field. The ratio of maximum to minimum flux for this channel is 3.7 during the interval of largest magnetic perturbation. Spectral analysis of the magnetic field data shows that the magnetic perturbation had a 96-s period and a 5-&ggr; rms amplitude and was right elliptically polarized, with the major axis tilted about 30 ¿ from the dipole meridian plane. The direction of propagation was in the dipole meridian at an angle of about 15 ¿ to both the ambient field and the dipole axis. Cross correlations between the principal magnetic field variation and the various proton flux measurements indicate that the lowest energy channel showing modulation in the field-aligned detector (48--71 keV) lags the field by 180 ¿. Higher energy channels lag by smaller amounts. Comparisons of the observation with three available wave modes, namely, the drift wave, the MHD slow wave, and the bounce resonant interaction associated with a transverse wave, are carried out in detail. It is found that none of the wave modes can explain all the correlated particle and field oscillations satisfactorily.