Abstract 1. The localization of serotonin (5-hydroxytryptamine) immunoreactive cells was examined phylogenetically in the central nervous system of invertebrates. 2. Invertebrate species used were as follows: Hydra magnipapillata of Coelenterata, Bipalium sp. of Platyhelminthes, Neanthes japonica and Pheretima communissima of Annelida, Ligia exotica, Procambarus clarkii. Helice tridens and Gryllus bimaculatus of Arthropoda, Pomacea canaliculata, Aplysia kurodai, Achatina fulica and Limax marginatus of Mollusca, Asterina pectinifera of Echinodermata and Halocynthia roretzi of Protochordata. 3. Serotonin immunoreactivity was recognized in the central nervous system of most of these species. However, the immunoreactivity was unclear in Hydra, Asterina and Halocynthia. The distribution of positive cells became centralized in the central nervous system of A. kurodai, A.fulica and L. marginatus. 4. Serotonin cells in the central nervous system of invertebrates were suggested to have a tendency to aggregate and finally to centralize with the phylogenetical progress of the species.