In order to solve the question whether Plathelminthes belonging to the taxon Acoela have a brain and an orthogon of the common flatworm type, an immunocytochemical study of the pattern of serotonin (5-HT) in four species was performed. In all species the 5-HT immunoreactivity revealed no ganglionic cell mass typical for other Plathelminthes, only a symmetrical brain-like structure composed of commissural fibres associated with a few cell bodies. 5-HT immunoreactivity was detected in three to five pairs of longitudinal nerve cords, connected by an irregular network of immunoreactive transverse fibres. No regular orthogon was visualised. All the surface sensilla were strongly immunoreactive. The unique commissural brain and cordal nervous system found in Acoela support the view of a deep gap lying between Acoela and other Plathelminthes.