Characters of the nervous system have scarcely been used in consideration of the phylogenetic relationships of the Tricladida. In order to contribute characters toward this end, we studied the nervous system of Planaria torva with immunocytochemical methods and we compare the results with what is known of nervous system structure in other triclads. Antisera against four neuroactive substances were used: neuropeptide F (NPF), RFamide, GYIRFamide, and serotonin (5-HT). All four substances showed positive immunoreactions throughout the nervous system; and the patterns of distribution of the first three were similar, while 5-HT showed a distinct distribution. The organization of these nervous system elements in Planariidae and Dendrocoelidae resembles that of the Maricola but differs from that of the Dugesiidae. The results support the hypotheses that the Paludicola is a more advanced taxon than the Maricola and that the Planariidae and Dendrocoelidae together are a sister group of the Dugesiidae.