This paper discusses the problem of electron paramagnetic resonance (EPR) line shape of localized magnetic ions in conductors which exhibit a strong dependence of their electrical properties on the applied magnetic field. Semimetals and narrow‐gap semiconductors with highly mobile charge carriers are specifically considered. When specimens (in the form of a plane‐parallel slab) are much thinner than both the skin depth and the internal wavelength, the EPR line shape is Lorentzian, determined by the dissipative part of the dynamic magnetic susceptibility of the localized paramagnetic ions. When the specimens are thicker than the skin depth, a variety of line shapes is predicted, determined not by the value of the conductivity as such, but rather by the loss tangent (i.e., the ratio of the dissipative‐to‐dispersive components of the conductivity). This ratio, in turn, depends on the type of free carriers present (electrons or holes), carrier concentration, carrier mobility, as well as sample temperature and the value of the magnetic field at which EPR occurs. When the specimens are thicker than the internal wavelength, but thinner than the skin depth (a condition frequently satisfied in narrow‐gap semiconductors and semimetals), Fabry‐Perot–like geometrical resonances arise, which again profoundly affect the line shape. Specific ranges of the loss tangent, characteristic of narrow‐gap semiconductors and semimetals, are discussed analytically and illustrated by numerical calculations. Where available, EPR data involving transmission experiments in Hg1−xMnxTe are presented.