AbstractImmunoreactions (IR) in the nervous systems (NS) of species representing the platyhelminthic taxa Acoela, Proseriata, Tricladida and Rhabdocoela (Dalyellioida, Typhloplanoida, Kalyptorynchia) have been studied by means of the peroxidase‐antiperoxidase (PAP) immunocytochemical method and the whole mount immunofluorescence (FITC) method. Antisera to the molluscan cardioactive neuropeptide FMRF‐amide, to the bioamine 5‐HT and to the vertebrate neuropeptides bovine pancreatic peptide (BPP), leu‐enkephalin, substance P and human growth hormone releasing factor (GRF) were used. Perikarya and fibres positive to FMRF‐amide antiserum (anti‐FMRF‐amide) appear in the NS of all investigated species. Since IR to anti‐FMRF‐amide has been obtained in worms representing taxa of all flatworm classes it is suggested that substances with similar sequences have been inherited from the stem form of Platyhelminthes. IR to 5‐HT antiserum (anti‐5‐HT) observed in the NS of all the investigated flatworms confirm the importance of this bioamine, which may have appeared early in platyhelminth evolution. IR to GRF antiserum was found in the acoel, the triclad and the rhabdocoel species, IR to leu‐enkephalin antiserum in the triclad and rhabdocoel species and IR to substance P ant