The properties of parallel propagating longitudinal and transverse oscillations in isotropic, gyrotropic, uniform magnetized plasmas of arbitrary composition are investigated on the basis of Maxwell equations and the relativistic Vlasov equation. For equilibrium thermal plasmas the dispersion relations for parallel propagating transverse and longitudinal waves are determined. The relativistically correct solution of the dispersion relation of longitudinal plasma waves in an isotropic equilibrium electron plasma leads to two new effects unknown from the nonrelativistic dispersion theory. First, the number of damped subluminal modes is limited to a few (mode limitation effect), and secondly, for relativistic plasma temperatures the few individual modes complement each other in the sense that the dispersion relations ωR = ωR(k) continuously match each other (mode completion effect). The second effect does not occur at nonrelativistic temperatures.