A bstract. The virial theorem has been applied to the study of the equilibnum configuration and the stability of inter- stellar clouds. We find that for a given total mass and a given negative total energy the cloud has always a stable equi- librium configuration and that the interstellar clouds actually observed are in, or very near to, the state (specified by temperature, density and size) of the stable equilibrium. An equation governing the change of the equilibrium configu- ration through some slow processes of energy transfer with outside is derived and the effect of rotation on the equilibrium configuration is investigated. We have also generalized Ledoux's method of applying the virial theorem to the problem of pulsating stars, and have derived a general formula for the period of pulsating stars.