Intersatellite radio wave propagation in the lower-frequency branch of the extraordinary mode, or Z mode, has been studied using data recorded by the Isis I and Isis II topside sounder spacecraft during a rendezvous near the north pole. The working frequencies, near 1.0 MHz, lay between the gyrofrequency and the upper hybrid resonance frequency, the former being greater than the plasma frequency. In contrast to the 100-&mgr;s length of the emitted pulses, the received pulses were stretched to about 10 ms and had sharp rising edges. At any given time the range of transmitted frequencies for the Z mode was limited. The central frequency of this ionospheric passband and the time delay of the leading edge of the pulses both increased monotonically with time as the two-satellite geometry changed. These observations are in good agreement with the predictions of ray optics applied to a smoothly varying medium. However, the detailed pulse shape was quite unlike the theoretical shape predicted under the WKB approximation; this suggests strong scattering. Also, received pulses occasionally had quasi-sinusoidal modulation envelopes. As in previously reported results for the whistler mode, ionospheric irregularities appear to strongly influence the propagation of a wave that can have large refractive indices.