Using a high-resolution Fabry-Perot interferometer, we have studied the emission of a far-infrared p-type germanium cyclotron laser in a frequency range of strong emission (82–87 cm-1). A nonlinear tuning of the emission frequency with magnetic field, a magnetic field dependent width, and a splitting of the emission line (near 85.8 cm-1) were found. We present an analysis showing that the emission behavior can be understood by recognizing that the laser level separation depends not only on the magnetic-field strength, but also on the ratio of the electric and magnetic-field strengths. We further relate the observed line splitting to radiation absorption due to transitions between Zeeman-split states of shallow acceptors.