The electrostatic stability properties of a rotating, charge‐neutralizedP‐layer are investigated within the framework of a hybrid (Vlasov‐fluid) model in which the layer ions are described by the Vlasov equation, and the layer electrons and the uniform background plasma are described as macroscopic, cold fluids. It is assumed that thePlayer is thin, with radial thickness (2a) much smaller than the mean radius (R0), and that &ngr;≪1, where &ngr; is Budker’s parameter for the layer ions. Electrostatic stability properties are calculated for perturbations about a weakly diamagneticPlayer with rectangular density profile, described by the equilibrium distribution functionf0b= (nbR0/2&pgr;mi) &dgr;[H−VzPz−mi(V20−V2z)/2]&dgr; (P&Vthgr;−P0), whereHis the energy,P&Vthgr;is the canonical angular momentum,Pzis the axial canonical momentum, andnb,R0,Vz,V0, andP0are constants. The stability analysis is carried out including the effects of a uniform background plasma, and weak self‐magnetic fields. Although a slow rotationalPlayer (P0≳0) is found to be stable, it is shown that a fast rotationalP‐layer (P0<0) is unstable for sufficiently high background plasma density (&ohgr;2p≫&ohgr;2ci). The typical instability growth rate is a substantial fraction of the ion cyclotron frequency.