Rohon-Beard neurones show substance P-like immunoactivity in their somas and in their centrally projecting axons. Peripherally, the morphology of their free nerve endings within the trunk skin has been shown using horseradish peroxidase staining. The excitation of Rohon-Beard neurones by natural and electrical stimulation of the skin has been examined using intracellular micro-electrodes in the late embryo of Xenopus laevis. Rohon-Beard cells are sensitive to transient, local indentation of the trunk skin, responding with one or a few impulses. They adapt rapidly to repeated stimulation. They can also be excited by a brief current pulse to the skin. They are not sensitive to slow indentation of the skin, nor are they excited by epithelial action potentials. The responses to skin stimulation are not abolished by a Ringer solution containing 12 mM-Mg2+ and only 0.5 mM-Ca2+. Intracellularly evoked action potentials in single Rohon-Beard cells are sometimes sufficient to evoke sustained episodes of fictive swimming. The results indicate that Rohon-Beard cells are responsible for detecting light touch stimuli to the embryo's body and for initiating swimming in response to this stimulus.