We have investigated the behavior of a terahertz quantum cascade laser in an external magnetic field. A reduction of the threshold current density and a simultaneous enhancement of the laser emission intensity are observed. Although several mechanisms can induce this effect, the suppression of nonradiative Auger-intersubband transitions through Landau quantization of the in-plane electron motion is the most probable candidate. In addition, the injection rate via resonant inter-Landau-level transfer and the waveguide properties are modulated by the field. We also observed clear shifts of the emission spectra when the external magnetic field is applied, while operating the device at constant voltage or current. (C) 2003 American Institute of Physics.