Balloon-borne X ray measurements with high time resolution were carried out in the South Atlantic magnetic anomaly to investigate the fine structure of the precipitation in this region. Measurements were obtained on two balloon flights launched from Cachoeira Paulista, S¿o Paulo, Brazil, on January 26 and February 1, 1994, and were supported by onboard electric field and geomagnetic field ground data. In one flight, we have found that rapid changes do occur in times of about 0.05 s during periods of a few seconds. They should be considered as the first evidence of the presence of microbursts in the low L value region of the South Atlantic anomaly. These microbursts were not correlated with sferic signatures in the same hemisphere and may be correlated with magnetic fluctuations in the geomagnetic field. Energy-time features of the microbursts are examined in order to identify possible responsible mechanisms. ¿ American Geophysical Union 1996