ABSTRACT A variety of vertebrate and invertebrate sensory cells are known to be sensitive to acetylcholine (ACh), although the purpose of ACh receptors on these cells is not understood. Acetylcholine elicits action potentials from the sensory cell of the crayfish stretch receptor organ, which receives inhibitory but not excitatory synaptic input (Wiersma, Furshpan & Florey, 1953). Among the vertebrates ACh causes spiking in several visceral and somatic sensory cells, including chemoreceptors and baroreceptors of the carotid body (von Euler, Liljestrand & Zotterman, 1941 ; Diamond, 1955), mechanoreceptors in the skin (Brown & Gray, 1948) and pain receptors (Skouby, 1951). The experiments to be described here demonstrate that ACh depolarizes the median photoreceptor of the giant barnacle, and test the possibility that the photoreceptor receives cholinergic synaptic input.