A genetic analysis realized, on Tunisian coasts, within and between two populations of a Teleostean (Sardina pilchardus) on one hand, and two populations of their parasitic Copepods (Peroderma cylindricum) on the other hand, reveal that(l) the 14 loci analysed are monomorphic in the Teleostean populations; (II) 3 loci among 13 investigated are polymorphic in the Copepod populations. Statistical tests from deviations of Hardy-Weinberg equilibrium and genotypic homogeneity show the existence of strong gene flows within and between Copepod populations. Thus, it seems that the Teleostean (Sardina pilchardus) and the Copepod (Peroderma cylindricum) constitute at least on Tunisian coasts, two homogeneous taxinomic units.