AbstractThe 5‐HT (serotonin) distribution in the nervous system of the macrodasyoid gastrotrichTurbanella cornutawas studied using immunocytochemical methods. Positive immunoreaction was found in two pairs of neurons. The neurons of one pair had processes which extended peripherally to the surface of the body. Central processes of both pairs entered the brain commissure and proceeded into the longitudinal cords. Unlike other acoelomate worms studied so far, the Platyhelminthes and the Nematoda, in this gastrotrich no 5‐HT positive perikarya or processes were present in the pharynx innervation, and no positive neurons sent processes directly to the nerve c