A monthly sampling of eels in the mixohalin lagoon of Mauguio (Languedoc, France) allowed the analysis of Anguillicola crassus (Nemathelmintha), Myxidium giardi (Myxozoa) and Eimeria anguillae (Sporozoa) population structures. This study showed that the high intensities of A. crassus, a parasite recently introduced in this area, seem to induce the development of both protists. However, any negative effect of these parasites of Anguilla anguilla can be detected by the study of the host size/weight relation.