Balloon borne corona probes designed to measure vertical electric fields have yielded data on the vertical electric structure of four thunderstorms studied during the Thunderstorm Research International Program (TRIP), 1981. All four thunderclouds exhibited a bipolar charge structure with an upper positive charge region and a lower negative charge region. The negative region was consistently centered between to 0 ¿C and -10 ¿C isotherm altitudes. The altitudes of the concentrated positive regions varied considerably. The average vertical extent of the negative regions was 1 km with estimated average charge concentrations ranging from -0.7 nC/m3 to 1.8 nC/m3. The average thickness of the positive regions was 1.5 km with estimated charge concentrations averaging 0.5 nC/m3 to 1.7 nC/m3.