Intracellular recordings were made from neurons of the trigeminal sensory ganglia of young adult sea lampreys. Receptive fields were mapped, and four classes of sensory cells were identified. Touch cells gave rapidly adapting responses to indentation of the skin. Pressure cells gave slowly adapting responses to indentation of the skin. Pit organ cells gave slowly adapting responses to mechanical stimulation of single lateral line pit organs. Nociceptive cells gave slowly adapting responses to destructive stimuli, such as puncture or burning of the skin. The axons of nociceptive cells had longer refractory periods and slower conduction velocities than did the axons of the other types of cell, indicating that nociceptive cells had smaller diameter axons.