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International audience; In the Mediterranean, the sea bass Dicentrarchus labrax is parasitized by two Monopisthocotylean monogeneans : Diplectanum aequans and Diplectanum laubieri. These congeneric species live on the gills of this fish. New data concerning the structure of their populations are presented. Analysis of the distribution of these parasites in a sample of hosts was used to propose general mathematical models. The lognormal model was thus found to fit the distribution of D. aequans, whereas that of D. laubieri can be described by a negative binomial model. More detailed analysis of Diplectanum cohorts on each host led to : 1) more accurate definition of the differences between the dynamics of these two monogeneans. D. aequans appeared as the predominating species; 2) determination of the importance of certain explanatory variables in relation to the dynamics of recruitment : the age and sex of fishes influence parasitism. Both parasites infect their host throughout the year, but fluctuations were observed linked mainly to the behaviour of D. labrax ; certain females were more parasitized, particularly during the spawning period. When studied in detail at infrapopulation level, these monogeneans appear as good tags of host ethology. Finally critical remarks are made with regard to some problems raised in population biology of parasitic helminths (frequency distribution, sampling strategy...).