In situ measurement of electron density height profile of the equatorial nighttime ionosphere, under a developing spread F event, was carried out using a dual mode high frequency capacitance (HFC) probe that was flown on board a SONDA-III rocket launched from Natal on December 11, 1985. This represents the first measurement of plasma bubble characteristics using an HFC probe. A series of plasma bubbles in varying degrees of their growth phase was detected mostly during the upleg of the flight. A somewhat detailed discussion, and comparative studies, of the measurements at the two probe frequencies (6.17 MHz and 11.75 MHz) are presented in this paper. Among the important findings from the present study is an evidence of electron temperature enhancement in the plasma bubble possibly caused by electron heating from energetic particle precipitation in the South Atlantic magnetic anomaly. ¿ American Geophysical Union 1991