The propagation of a shock wave, originating in a stellar interior, is considered when it approaches the surface of the star and assumes a self‐similar character, “forgetting” its initial conditions. The flow behind the shock is assumed to be spatially isothermal rather than adiabatic to simulate the conditions of large radiative transfer near the stellar surface. The adiabatic and isothermal flows behind such a shock are compared. The exact shock‐propagation laws, obtained by solving the equations in similarity variables, for different values of the parameter &dgr; in the undisturbed density law,&rgr;0∞&dgr;, and&ggr;, the ratio of specific heats, are compared with the approximate values calculated by Whitham's characteristic rule and the two show a generally good agreement.