In this paper we consider the spherical shape instability for a gas bubble in water subject to an ultrasonic pressure field. The radial oscillations are expressed through a perturbative solutions of the Rayleigh-Plesset equations and the distortions of the spherical shape are given by a superposition of spherical harmonics. The frequency regions corresponding to the first and the second ultraharmonic resonances of the radial oscillations are then explicitly considered. Under these conditions, the dynamic behavior of the bubble surface is described in terms of the Hill equation. The instability threshold can be obtained as a function of the excitation intensity. The easiest excitable surface mode is the 1⁄2 subharmonic of the radial oscillations.
