Objective of research: The purpose of the study was to study the phylogeny and evolution of trematodes of the order Schistosomatida (Plathelminthes: Trematoda) - endoparasites of animals and humans. Materials and methods. Samples of adult specimens and 5 larva species were collected and examined: Schistosoma turkestanicum Skryabin, 1913, Bilharziella polonica (Kowalewsky, 1899), Trichobilharzia ocellata (La Valette, 1854), Dendritobilharzia loossi Skryabin, 1924 and Gigantobilharzia acotylea Odhner, 1910. We were exploring their life cycles under both natural and experimental conditions. Adult forms of trematodes were identified by standard methods. The determination of cercariae produced by aquatic mollusks (Lymnaeidae, Planorbidae, Physidae, Melanoididae) was performed according to the studies. For a comparative analysis of morphological and biological features of the Tricladida turbellarians, we used materials from well-known manuals and monographs. Results and discussion. Based on the original study of morphology, biology, and life cycles of the five species of blood flukes - Schistosoma turkestanicum, Bilharziella polonica, Trichobilharzia ocellata, Dendritobilharzia loossi, Gigantobilharzia acotylea and the analysis of literature data on the phylogeny and evolution of the order Schistosomatida, a hypothesis of the origin of dioecy of schistosomatids was proposed. There is controversy about the phylogeny and evolution of representatives of the families Schistosomatidae and Bilharziellidae - parasites of endothermic vertebrates, including humans. The idea according to which dioecious and hermaphroditic state of flukes is primary and associated with morphological organization of free-living progenitors which the appropriated signs are inherited from. The above-mentioned opinion enables to explain the dioecy of trematodes of the order Schistosomatidae, their phylogeny and evolution
