The organization of the nervous system of Dendrocoelum lacteum (Tricladida, Paludicola, Dendrocelidae) and Polycelis tenuis (Tricladida, Paludicola, Planariidae) was studied by immunocytochemical double staining, using neuropeptide RFamide and serotonin (5-HT) antisera on cryosections. The study confirmed the status of the main nerve cords (MCs) as the most important and stable of the longitudinal cords and supported the hypothesis of a common phylogenetic origin of the MCs in flatworms. The ganglion-like structures along the MCs at the beginning of transverse commissures and lateral branches showed a close contact with ventral fibres of the submuscular nerve plexus indicating an origin from crossing points of insunken ring commissures. The distributional pattern and morphology of the RFamide-IR cell bodies in D. lacteum corresponded to that of neurosecretory cells. Most RFamide-IR cells were unipolar and rounded while 5-HT-IR cells were uni- bi- and multipolar. The neuropile consisted of a dense RFamide-IR and a loose 5-HT-IR network. RFamide dominated in all parts of the genital plexus.