A large number of spectral structures are observed by far infrared magneto-photo-conductivity in n-GaAs at 4.2K biased above impurity breakdown in a regime where current flow is filamentary. Phase changes are found in the center of the 1s−2p+1 line and new structures are observed which shift to higher magnetic fields with increasing bias voltage. The large strength and the intensity dependence of the photosignals indicate that the observed phenomena are due to self-organizing effects of the filament. Small variations of the electron population by infrared irradiation may cause substantial rearrangements of the spatial current distribution.