Strong dayside long-period electric and magnetic field fluctuations were measured on board the geostationary satellite GEOS 2 on April 11, 1979. The electric and magnetic field spectra obtained throughout this day clearly exhibit discrete frequency bands, lasting simultaneously for about 5 hours. The lower-frequency pulsation with a typical period of 260 s is only present in the electric field data (with typical amplitudes of 0.3 mV/m), whereas the higher-frequency oscillation dominates the transverse magnetic field spectra (typical period 95 s, typical amplitude 1 nT). The periods of both pulsations increase toward the dusk sector. The observed periods of the pulsations in conjunction with the measured equatorial electron densities, the ratio of the electric and magnetic field amplitudes for both frequency bands, and the constancy of the period ratio yield strong evidence that the two frequency bands represent fundamental and second harmonic mode eigenoscillations of geomagnetic field lines. The mean ratio between the shorter and longer periods derived from the measurements (about 0.35) is slightly different from theoretically obtained ratios, thus indicating shortcomings of currently available models. The phase relationship between the electric and magnetic field fluctuations of the higher-frequency pulsation, the direction of the wave Poyning vector, and the occurrence of the event after the spring equinox indicate that the major part of the wave energy is fed into the oscillation in the southern hemisphere and not close to the equatorial plane.