On January 26, 1994, a coordinated campaign was conducted to study the stratospheric conductivity in Brazil. The measurements were obtained, each 10 min, by a double probe electric field detector during a stratospheric balloon flight launched at 0720 LT from Cachoeira Paulista, Brazil (geographic coordinates 22¿44'S, 44¿56'W). The data were supported by radar and Meteosat satellite images. The balloon reached the ceiling altitude of 29.2 km in 1 hour and 20 min after launch and then floated for about 7 hours. For the first time, positive and negative conductivity vertical profiles were obtained in Brazil. During the flight, at an average altitude of 27.5 km, the balloon passed over two thunderclouds as identified by onboard video images and photographs, as well as by radar data. The simultaneous conductivity measurements and thundercloud observations allowed to determine that the presence of clouds affects the stratospheric negative conductivity, producing an extended and cumulative drop after the balloon passed over each cloud. The positive conductivity, in turn, was found to be predominantly influenced by the photoelectric effect. ¿ 1999 American Geophysical Union