A linear description of the temporal evolution of a localized wave packet in a weakly inhomogeneous plasma is derived by expanding the spatial variations in the plasma parameters up to second order. The solution applies to systems with or without unstable normal modes, and it can provide a quantitative measure of the correlation length of the fluctuations to be expected, as exhibited by the spreading and propagation speed of the wave packet. The general formalism is applied to the collisionless trapped-particle mode in tokamak geometry, the current-driven drift mode, and the negative-energy loss-cone mode in a magnetic-mirror geometry.