The anvil of a supercell severe storm in central Oklahoma was penetrated by two free balloons carrying corona probes to measure electric fields. The two balloons passed through the portion of the anvil upstream of the precipitation core and provided information on the charge concentrations and thicknesses of the electric charge regions within the cloud. The trajectories of the balloons relative to the storm were horizontally separated by approximately 35 km, providing information on the horizontal extent of the charge regions. Contrary to the general assumption that anvil interiors are positively charged, the results of the two flights indicate that extensive regions of both net positive and negative charge were present in the interior of the anvil. Net positive charge was observed in both balloon flights in the lower portion of of the anvil interior. The verticle thickness of the positive charge region was measured in each flight to be 1.6 and 2.3 km, respectively. A charge concentration of +0.2 nC m-3 was estimated from one of the flights. Net negative charge was observed by both balloons in the upper portion of the anvil interior. The vertical thickness and charge concentration of the negative charge region were estimated from one flight to be 3.2 km and -0.1 nC m-3, respectively. Screening charge layers were identified at the upper and lower boundaries of the anvil, with a typical thickness of a few hundred meters, which is an order of magnitude greater than that calculated in a previous model. We propose a conceptual model of the development of anvil charge regions based on our balloon measurements and previous observations of anvil dynamics. In this model we suggest that the thicknesses of the anvil screening layers increase with distance from the precipitation core of the storm owing to turbulent mixing near the cloud boundaries and differences in the sedimentation rates of the cloud particles. Negative charge in the upper portion of the anvil interior results from the expansion of the upper negative screening layer into lower levels of the anvil cloud. ¿ American Geophysical Union 1989