Parasitological data are increasingly used to provide information on host populations, trophic interactions and free-living biodiversity. Here, parasitic helminths have been researched in Sardina pilchardus and Engraulis encrasicolus from the Bay of Biscay (North-East Atlantic). In addition, a review of helminths infecting E. encrasicolus has been provided. Surprisingly, our field study reveals the occurrence of only three species of Ascaridoidea (Anisakis simplex sensu stricto, Anisakis pegreffi and Hysterothylacium aduncum) in both fish species despite the high diversity of helminth parasites (mostly heteroxenous trophically-transmitted) recorded throughout their distribution range, i.e. 39 and 15 taxa, respectively, for S. pilchardus and E. encrasicolus: among them, 12 shared taxa. This low diversity of helminth parasites in the Bay of Biscay may be indicative of a low free-living biodiversity. Moreover, similarities in the helminth parasites of S. pilchardus and E. encrasicolus, recorded both in our field study and in their entire geographical distribution, suggest ecological and feeding similarities between these two planktivorous fish. However, the higher prevalence and mean intensity of Ascaridoidea in E. encrasicolus (vs. S. pilchardus) in the Bay of Biscay also highlighted differences, which were considered the result of its higher trophic plasticity.