Quantum-mechanical considerations on negative-effective-mass carriers in crystals show the following: The energy-level structure of particles with negative effective mass, in the presence of a magnetic field, consists of a system of inverted Landau levels. A simple derivation of the level inversion is given. Sharp contradistinctions are found on the behavior of the levels by comparison to that of positive-mass regions of momentum space. The significance of this type of level structure for cyclotron resonance transitions, in particular in the case of the observed small negative-mass branch of heavy holes in Ge is discussed.