The general dispersion relation for slow waves propagating in plasmas of arbitrary &bgr; is discussed in detail along with some possible applications to the magnetosphere. It is shown that the electromagnetic corrections to the quasi-static approximatin are important only at frequencies below the lower hybrid frequency when the parallel phase velocity is greater than the electron thermal speed. Next, a comparison between the properties of these slow waves and those of the fast electromagnetic ones is given. Adopting an ordering scheme in the frequency range close to the ion gyrofrequency appropriate to the physical parameters of interest, we show that the perpendicular phase velocity of the fast electromagnetic waves should be at least 1 order of magnitude greater than the thermal velocity, which imposes very stringent propagation conditions on the properties of the whistler and ion cyclotron waves close to the particle gyrofrequency. Applications to the magnetosphere are discussed. It is shown that it is almost impossible for the whistler mode or the electromagnetic ion cyclotron mode to be amplified by instabilities associated with the high perpendicular thermal velocities characteristic of the magnetospheric plasma. Finally, a new process concerning the ducting of the whistler mode along the geomagnetic field lines is proposed.