The ontogeny of serotonin (5-HT)-immunoreactive (IR) cells in the gut epithelium of an oviparous elasmobranch, Scyliorhinus torazame, was examined immunohistochemically. 5-HT-IR cells first appeared in the proximal part of the vitellointestinal duct (VID) and in the anterior part of the midgut of the embryo (30 mm in total length). At the 40-mm stage, the cells slightly increased in number and spread to the posterior part of the midgut, but no labeled cells were found in the foregut or hindgut. By the late embryonic (74- and 80-mm) stages, 5-HT-IR cells were markedly increased in number in the spiral intestine and stomach, whereas they were few in the VID and rectum. During these stages, the density of the cells in the inner yolk sac, the derivative of the VID, tended to be increased. This tendency seemed to be consistent in the posthatching juveniles at the 95-mm stage. In juveniles, 125 mm in length and 1.7 months after hatching, the cells further increased in number in the spiral intestine, reaching their adult value. Double immunostaining by the use of anti-5-HT and -neuropeptide Y (NPY) antisera demonstrated that some of the 5-HT-IR cells were also positive for NPY.