The blind cave salamander''s (Proteus anguinus L.) sensitivity for electrical fields was determined by recordings from afferent fibers coming from ampullary electroreceptors. The animals were stimulated with single rectangular or continuous sinusoidal electrical signals passed through the cave water in the experimental tank. Best threshold sensitivities of the fibers for both kinds of signals were in the same range (0.1–0.5 mV/cm = 30–150 nA/cm2). The fibers showed phase-locked responses to rostrally cathodic phases of rectangular or sinusoidal stimuli and suppression to anodal phases. Units were tuned to ''best frequencies'' (B.F., sine waves) between 20 and 40 Hz. Any functional, i.e. phase-related, firing or suppression of firing occurs in a total frequency range of less than 0.1 to around 100 Hz. This frequency range of tuning in the primary fibers was the same as found in the behavior, and this therefore reflects the peripheral sensory properties. Electrical sensitivity might be of special value for these cave dwelling animals, as they may detect and recognize prey animals by means of the prey''s electrical field spreading into the surroundings, i.e. its electromyog