Pulse trains superimposed on the initial part of the electric field change of positive ground flashes are investigated. The characteristics of these pulse trains are found to be different from those of negative ground flashes and cloud flashes. Each pulse included in a pulse train appears as a bipolar waveform with pulse width from 5 to 52 μs and a mean of 18 μs. The time intervals between successive pulses are 10 to 180 μs with a mean of 54 μs. For each bipolar pulse, the ratio of the peak amplitude of the initial polarity to the maximum overshoot amplitude ranges from 0.4 to 4.7 with a mean value of 1.3. The ratio of maximum peak amplitude of successive pulses to the amplitude of the following return-stroke peak ranges from 0.02 to 1.9 with a mean of 0.27. ¿ 1998 American Geophysical Union