From an attentive study of the largest number of available publications, we tried to establish the list of fish species harbouring fully mature Acanthocephala, i.e. females with completely developed eggs. This character is the sole on which we are allowed to assure that this fish is truly a definitive host for the parasite. This very preliminary work shows that acanthocephalan specificity for the definitive host is narrower than it was generally said. If few so called "primitive" Acanthocephala are indeed parasitic in "primitive" fishes, in the majority of cases the main factor for infestation is related to the trophic behaviour of the definitive host and do not depends on its phylogenic position.