The complex electric HF-conductivity σ of a plasma in a magnetic field has been calculated on the basis of the BOLTZMANN transport equation. Negative values of the real part of σ can occur, if the system is in a state of overpopulation, i. e. if ∂f0/dp > 0. where f0 is the momentum distribution function of the free carriers. Furthermore, the collision frequency or the mass of the carriers must be dependent on the momentum p.