The time-dependent growth rate for parallel propagating elecromagnetic cyclotron waves is derived for a magnetoplasma which is characterized by a time dependent compressional perturbation superimposed on an equilibrium configuration. Such perturbations are commonly observed in the Earth's magnetosphere as a consequence of resonant field line oscillations, solar-wind disturbances, and other phenomena. The time dependencies of the magnetic field, thermal plasma density, energetic particle distribution function, and resonance condition are first related through a single dimensionless time parameter b(t) using the ideal MHD assumption. For cases in which the particle distribution can be described by F(&agr;, E)=f(E)sin&agr;(E)&agr;, the time dependent wave growth rate is then given by &ggr;≂&ggr;0(1+&Ggr;) where &ggr;0 is the equilibrium growth rate and &Ggr;(b) is a function of the equilibrium parameters and the time parameter b. The term ‖&Ggr;‖ is generally small compared to 1, and the effect is a small modulation of the equilibrium growth rate by &Ggr;. If the particle distribution is locally near marginal stability, however, ‖&Ggr;‖ is large compared to 1, and the growth rate modulation can be much larger than for a distribution which is not near marginal stability. The results suggest that particle populations which are near marginal stability may be strongly influenced by perturbations in the magnetic field and plasma. Marginally stable distributions may thus play an important role in magnetospheric dynamics as well as determination of radiation belt characteristics. ¿American Geophysical Union 1990