In the Etang de Thau (Herault, France), three species of Helicometra parasitize in their adult stage the digestive tract of the black goby Gobius niger, the grass goby Zosterisessor ophiocephalus, the grey wrasse Symphodus cinereus, and the common eel Anguilla anguilla (Teleostei). These fish act as definitive hosts in the biological cycle of these platyhelminths. Also H. gobii exists in G. niger, Z. ophiocephalus, S. cinereus and A. anguilla ; H. fasciata is found in G. niger ; and H. pulchella is only found in S. cinereus. In the case of the two gobies and the eel, these three congeneric digenes sometimes coexist in the same infracommunity. Spatio-temporal analysis of these different populations has enabled : 1) detection of seasonal variations in their structure, 2) identification of the relationships between the nature of their specificity and demographic strategy, and 3) the global absence of statistical dependence between the intensities of different species in the same infracommunity to be show. Implications arising from previous research concerning the demographic behaviour of these trematodes in their first intermediate host (mollusc compartment), were taken into account when interpreting structures observed in the definitive hosts. The problems of coexistence between these species, which are taxonomically very close in the same parasite community, and the under-lying speciation mechanisms are discussed in the light of population structuration.