Gymnophallids are parasites of coastal birds and use marine bivalve molluscs as first intermediate hosts and molluscs (bivalve or gastropod) or polychaetes as second intermediate hosts. The so-called germinal sacs, which produce cercariae and metacercariae, have been described for six gymnophallid species but confusion has existed as to their nature and if the host in which they were found is the first or second one (Szidat, 1962; James, 1964; Chubrik, 1966; Tsimbaljuk et al., 1978; Ching, 1982). Our experimental work on Cercaria margaritensis Ching, 1982 confirmed that the germinal sacs are found in the second intermediate host and that the sporocysts develop in the bivalve Turtonia minuta (Galaktionov, 1996). Further investigation has shown that Cercaria margaritensis Ching, 1982 represented a new species Parvatrema margaritense Galaktionov, Irwin et Saville, 2006. Daughter sporocysts of P. margaritense produce typical gymnophallid furcocercariae that are shed from T. minuta and penetrate the prosobranch Margarites spp., the second intermediate host (Fig. 1). Each cercaria migrates to the extrapallial cavity, drops its tail, and changes into a germinal sac considered as a parthenogenetic metacercaria (PM). This pri- mary parthenogenetic metacercaria (PM1) produces the second generation of metacercariae (PM2) in its brood sac. The PM2 metacercariae leave the PM1 and parasitize the extrapallial cavity of the M. helicinus. The PM2 are also parthenogenetic and produce the third generation of metacercariae (M3) which are infective to the definitive host. The birds are infected when they eat molluscs containing PM2 with mature M3 inside them. Life cycle transmission in other gymnophallids with parthenogenetic metacercariae appears to follow a similar scenario. At present the life cycle described above seems to be one of the most complex among trematodes and includes two periods of parthenogenetic re- production. The first takes place in the first intermediate (molluscan) host on the stages of mother and daughter sporocysts and results in production of a huge number of intective stages (cercariae). This is typical for trematodes. Proc. Zool. Inst. Russ. Acad. Sci, 310. 2006: 51-58 Phenomenon of parthenogenetic metacercariae in gymnophallids and aspects of trematode evolution Kirill V. Galaktionov Zoological Institute, Russian Academy of Sciences, Universitetskaya nab., 1, St.Petersburg, 199034, Russia