Atmospheric electric potential gradient measurements were made at two localities simultaneously during pyroclastic flow events on September 25, 1993 at Unzen volcano. Immediately after generation of the pyroclastic flows, the potential gradients showed a positive anomaly that was followed by a negative anomaly. The magnitude of the positive anomalies were nearly equal at the two measurement localities, whereas the magnitude of the negative anomalies at the measurement locality, close to the axis of the ash cloud, was larger than the magnitude at the off-axis measurement locality. These differences may be accounted for by the differences in the distance between the ash cloud and each measurement locality. It is suggested that the height to the positive charge was sufficiently larger and the height to the negative charge was sufficiently smaller than the horizontal distance between the two measurement localities. Our results suggest that the upper part of the ash cloud, which contained volcanic gases and/or relatively fine ash, carried a net positive charge and the lower part of ash cloud, which contained relatively coarse ash, carried a net negative charge. This charge distribution in the ash clouds generated by pyroclastic flows is more consistent with the results reported for Vulcanian plumes from Sakurajiama volcano [Lane and Gilbert, 1992; Lane et al., 1995> and Asama volcano [Hatakeyama and Ishikawa, 1946> than any other cases. ¿ American Geophysical Union 1996