The stability of a cylindrically symmetric plasma envelope driven outward by blast waves is considered. The plasma fluid is assumed to be a compressible, isentropic gas describable as an ideal gas ( p=a&rgr;&ggr;, &ggr;≳1). The stability problem of such an envelope undergoing self‐similar motion is solved by considering the initial‐value problem. It is shown that in the early phase of an expansion, the envelope is unstable to Rayleigh–Taylor modes which develop at the inner surface. In the later phase of the expansion, the Rayleigh–Taylor modes are weakened due to the geometrical divergence effect. The implications of the time‐dependent behavior of the Rayleigh–Taylor instability for plasma switches are discussed.