Evoked potential and unit responses from the lamprey brain to weak electric fields demonstrate that lampreys have an electrosensory system as sensitive as those of other electroreceptive fishes. Electrosensory responses were recorded in the dorsal medulla, the midbrain torus semicircularis, and the optic tectum. Similarities in the structure of the anterior lateral line nerves and medullary organization between lampreys and several primitive jawed fishes indicate that the electroreceptive systems are homologous in these taxa. Thus electroreception was probably present in the earliest vertebrates ancestral to both agnathans and gnathostomes.