A quantum cascade laser emitting at 1.39 and 2.3 THz is demonstrated. The structure is based on a large single quantum well and multiwavelength operation is obtained by selectively injecting carriers by resonant tunneling in the excited states of the well. A magnetic field applied perpendicularly to the layers increases the gain enabling laser action. Extremely low values of the threshold current density (below 30 A/cm2) are observed for both wavelengths. A study of the dependence of the laser emission intensity as a function of the applied magnetic field is also presented.