A theory is put forth to account for the acute sensitivity of marine elasmobranches (cartilaginous fishes) to exogenous electric fields. First, morphological adaptations cause a significant fraction of the voltage drop in the vicinity of the fish to occur across the epithelium of a specialized receptor organ, the ampulla of Lorenzini; that is, the stimulus is focused. Second, this stimulus modulates the repetitive discharge of the primary afferent fibers which innervate the ampulla. Third, these fibers converge centrally to integrate the outputs of many ampullae and markedly increase the signal-to-noise ratio of the process. Simple quantitative estimates are given for each of these steps, and it is shown that the predicted limits of electroreception are close to those actually observed. >