Abstract: In this paper, an analytical solution is presented for the Rayleigh-Taylorinstability of a viscous liquid drop at high Bond numbers. The instabilityplays an important role in the early stage of the aerodynamic breakup ofliquid drops in a supersonic flow. Based on the analytical solution, themaximum size of a liquid drop stable with respect to Rayleigh-Taylor instability and theinitiation time of drop breakup can be predicted. The predictions show goodagreement with relevant experimental and analytical results from literature.