International audience Different congeneric species of the genus Ligophorus are ectoparasites of mullets (Teleostei). Several species of Ligophorus which are strictly specific can coexist on the same host species. Due to the host migration between the sea and lagoons, these parasites guilds are subject to fluctuating environmental constraints. The nature of the phenotypic variability of L. imitans has been analysed. This study concerns an isolated population (Rhône delta, Camargue, France) of L. imitans, parasite of Liza ramada. A biometric analysis using image processing techniques is developped. Fourteen biometric variables concerning the sclerotized parts (opisthapteur and genitalia) of this platyhelminth have been considered. The phenotypic variability has been analysed using a principal component analysis. This population displays a great morphological and anatomical plasticity, but in a continuum. The different parts of these organisms cannot be described by simple and generalised allometric growths. The correlation analysis between the different variables reveals in particular that the genitalia is morphologicaly independent of the haptor.