The spectrum of immunomodulating molecules produced by tapeworms is not yet well understood. The aims of this study, on the tapeworm Diphyllobothrium dendriticum, were: 1) detection and quantification of prostaglandins (PGs) E-2 and D-2 by high performance liquid chromatography; 2) visualization of PGE(2) and PGD(2) in specific cells, using methods of immunocytochemistry and confocal laser scanning microscopy; and 3) investigation of the ultrastructure of the cells potentially producing PGE(2) and PGD(2). The PGE(2) immunoreaction (IR) was found in the apical terminals of the frontal glands and sensory organs in the tegument and in small neurons belonging to the main cords and commissures. PGE(2)-IR partly coincided with alpha-tubulin-IR. PGD(2)-IR occurred in the muscle fibers of longitudinal and transverse body muscles and coincided with phalloidin TRITC staining. Both PGE(2) and PGD(2) were found in the flame cells of the excretory system. Ultrastructural study of the tegument revealed two types of structures that potentially produce PGE(2): ciliated and unciliated free nerve endings and frontal gland terminals reinforced with neurotubules. In the main nerve cords, small neurons were identified as potentially exhibiting PGE(2)-immunoreactivity. In homogenates of the plerocercoids, the measured content of PGE(2) and PGD(2) was 33.15 ng mg(-1) and 1.94 ng mg(-1) of fresh tissue weight, respectively. We found evidence of PGE(2) and PGD(2) in D. dendriticum parasitizing Coregonus autumnalis (fish) and proved excretion of PGE(2) and PGD(2) in response to C. autumnalis blood serum. Prostaglandins produced by D. dendriticum probably play a dual role: 1) PGE(2) and PGD(2) potentially modulate the fish antiparasitic immune response; 2) PGE2 is presumably necessary for proper development and function of the nervous system, and PGD2 can act as an antagonist against mediators causing muscle contraction. (C) 2017 Elsevier B.V. All rights reserved.