Specializations for electroreception in sense organs and brain centers are found in a wide variety of fishes and amphibians, though probably in a small minority of teleost taxa. No other group of vertebrates or invertebrates is presently suspected to have adaptations for electroreception in the definition given here. The distribution among fishes is unlike any other sense modality in that it has apparently been invented, lost completely and reinvented several times independently, using distinct receptors and central nuclei in the medulla. There are so far no clearly borderline or transitional fishes, either physiologically or anatomically. We rather expect a few new electroreceptive taxa to be found. The evoked potential method and the newly validated central anatomical criteria provide two useful tools for searching. Although Myxiniformes probably lack electroreception, it is well developed in Petromyzoniformes and in all other non-teleost fishes except Holostei. Thus Elasmobranchia, Holocephala, Dipneusti, Crossopterygii, Polypteriformes and Chondrostei have the physiological and anatomical specializations in a common form consistent with a single origin in primitive vertebrates. Amphibian ancestors probably inherited the system from a stem similar to one of these and passed it on at least to the ambystomatoid and salamandroid urodeles, apparently after losing the kinocilium of the sense cell. The suggestion of electroreception in ichthyophid apodans from skin histology has not been confirmed physiologically, behaviorally or by brain anatomy. With respect to more advanced fishes the most parsimonious interpretation is that the entire system, peripheral and central was lost in ancestors of holostean and teleostean fishes and new systems reinvented in Siluriformes, in Gymnotiformes, in Xenomystinae and in Mormyriformes. These 4 taxa must represent at least two, and probably 3 or 4 independent inventions, presumably from mechanoreceptive lateral line organs and brain centers.