1. We examined the physiological properties and distribution of electroreceptors in the skin of adult lampreys (vertebrates, class: Agnatha) by recording electroreceptor afferent fiber activity in the anterior lateral line nerve. Stimulation was with uniform or local electric fields in the water around the fish. 2. Lampreys possess ampullary electroreceptors widely distributed over the head and trunk (Fig. 1) that are sensitive to weak, low-frequency electric fields. The response thresholds to uniform fields are 1–10 μV/cm and the dynamic range of the receptors includes 4 log units of intensity (Fig. 3). Maximum sensitivity with sinusoidal currents is to frequencies ≦ 1 Hz (Fig. 4). 3. Like the ampullary receptors of elasmobranchs and other non-teleost fishes, lamprey electroreceptors are excited by weak cathodal fields (i.e. negative at the receptor opening relative to a distant reference) and inhibited by anodal fields. With very intense stimuli (1–10 mV/cm) the responses reverse so that strong cathodal fields are inhibitory and anodal fields are excitatory (Fig. 3). These results indicate that similar transduction mechanisms exist in the electroreceptors of lampreys and non-teleost jawed fishes. Likewise, as in other non-teleosts, all electroreceptors in lampreys including those on the trunk are innervated by the anterior lateral line nerve. 4. The similarities in receptor physiology and innervation taken in conjunction with known similarities in medullary organization indicate that electrosensory systems of lampreys and non-teleost gnathostome fishes are homologous.