Morphogenesis of the true ascocercus (euascocercus or euascocysticercoid) was studied in Schistotaenia srivastavai, a relatively rare parasite of the Red-necked Grebe Podiceps griseigena, and of intermediate hosts, damselfy and dragonfly larvae, in the Okhotsk-Kolyma region. Stages of postembryonic development were reconstructed by the material from spontaneously infected dragonflies' larvae, which corresponded to the published data on the development of metacestodes from the genus Tatria s. l. studied by Mrazek (1927) and Rees (1973). The euascocercus is the most widespread morphological modification of ascocysticercoids among Schistotaeniidae. According to Gulyaev (1989) and our data, the species of the family (the genus Mircia) have also a polycephalic modification of the ascocercus, or the multicercus. It represents a maternal individual that is filled with numerous small filial cysticercoids during the postembryonic development. These cysticercoids are formed of individual buds in the outer wall; later on, they are gemmated into the primary cavity of the maternal individual. Consequently, each daughter individual possesses its own single-layer exocyst, homological to the inner layer of the two-layered exocyst of the other ascocerci. Supposedly, exocyst's outer layer of monocephalic ascocerci is homologous to the outer wall of multicercus' maternal individual. Finally, diagnostic features of the third modification of ascocerci, namely megalocercus, described in the uniquely large metacestode Dioecocestus asper, corresponds to the characteristic of the metacestode Schistotaenia tenuicirrus, studied by Boertje (1975). S. tenuicirrus differs from D. asper in the spiral configuration formed on the entire surface of the endocyst (in D. asper, only on its dorsal side), and in a low number of proglottids in the larval strobile (D. asper possesses a multisegmental strobile). The reason, why so large protective envelopes (exo- and endocysts) develop in S. tenuicirrus with relatively small size of prospective body, remains unclear. Thus, life cycles of Schistotaeniidae represent all three modifications of the ascocerci: the true ascocercus (Shistotaenia, Tatria, Ryjikovilepis, Joyeuxilepis), the multicercus (genus Mircia), and the megalocercus (S. tenuicirrus).