A thresholdlike behavior of the far-infrared photoconductivity due to cyclotron resonance and a drastic deviation of the cyclotron-resonance line shape from a Lorentzian has been observed in n-GaAs at low temperatures by applying a high-power cw far-infrared laser. Both effects may consistently be explained in terms of generation-recombination-induced nonequilibrium phase transitions showing that, besides impact ionization of impurities, cyclotron resonance can critically control the conductivity of the semiconductors.