Auditory (electrophonic) and visual (phosphene) sensations were generated in human subjects through direct stimulation by low-frequency electric currents. The effects occurred at frequencies in and near the ELF (extremely low frequency) communication band. Frequency-response characteristics for both effects were measured. Both sensory modalities were found to behave much like frequency-selective filters but with a number of important nonlinearities. The minimum thresholds of current for the phosphene and auditory effects occur, respectively, at 18 to 22 Hz and 60 to 90 Hz. The effects were also generated by the application of two simultaneous sine waves well above the cutoff frequency when the difference frequency was near the minimum threshold. Habituation of the phosphene and of the auditory response was noted. These effects and difference-frequency effects received emphasis because of potential implications for the understanding of sensory processes. Potential diagnostic and therapeutic applications are described.