Acoustic data obtained from thunderstorm observations during 1975 near Socorro, New Mexico, and during 1976 at J. F. Kennedy Space Center, Florida, exhibited single, infrasonic pulses superimposed on the thunder signals arriving from the cloud in each of several thunder records. Only the Socorro data have been quantitatively analysed thus far. These data show infrasonic pulses with periods of about 0.5 sec and amplitudes of about 0.1 N/m2. We argue that these pulses were not generated by lightning channel heating. The pulse waveform is characterized by an initial compression followed by a rarefaction. Acoustic source reconstruction place the origins of these pulses in the cloud within essentially the same volume of space as the horizontal portions of the lightning channels. This volume is presumably the storage region for the charge participating in the lghtning events. If the pulses are electrostatic in origin as predicted by Dessler [1973>, then the data indicate a rapid (0.5 sec) intensification of the field prior to discharge.