Although the identity, projection patterns, and functions of serotonergic neurons in juvenile and adult Aplysia are relatively well understood, little is known about the development of these cells. We have used light and electron microscopic immunocytochemistry to investigate the genesis, differentiation, identity, and fate of the serotonergic cells in the embryonic, larval, and metamorphic stages of the life cycle of Aplysia. The results indicate that the first serotonergic cells emerge at midembryogenesis and that a total of five cells makes up the entire serotonergic system by hatching. These cells are part of a newly discovered ganglion in Aplysia, called the apical ganglion. This serotonergic system of five cells remains essentially intact throughout larval development. The apical ganglion, together with its serotonergic cells, is resorbed at metamorphosis. A distinct set of serotonergic cells, which begins to emerge by the end of the larval period, is rapidly elaborated during the metamorphic and early juvenile periods to form the adult serotonergic system. These results support the view that the larval and adult forms of the Aplysia nervous system consist of entirely distinct sets of serotonergic cells, each adapted to the stage-specific morphological and behavioral characteristics of the animal.