Contribution à l’étude du parasitisme du renard (Vulpes vulpes L.) par les Ixodidae (Acarina) dans le nord-est de la France. Interpretation de la dynamique saisonnière des parasites en relation avec la biologie de l’hôte. - Some 230 foxes were examined for collecting Ixodidae (Acarina). Six species occured, they were in decreasing frequency order: Pholeoixodes hexagonus (Leach 1815), Phloleoixodes canisitga (Johnston 1849), Ixodes ricinus (Linne 1758), Dermacentor reticulatits (Fabricius 1792), Phloleoixodes riigicollis (Schylze and Schlottke 1929), Ixodes acuminatus (Neumann 1901). The last species is regarded as an accidenta! one. The parasitic dynamics of Ixodes ricinits appears to be identical with results achieved by other authors by removing that tick from other mammals or by collecting it in its habitat by the ’blanket method’: spring and autumn activity in the north-eastern part of France, the microclimate of the fox s habitat and the nocturnal trips of fox explain the parasitism by Ixodes ricinus during high summer temperatures or during non-existent rainfall spells in spite of the sensibility of this parasite to desication. Under 48°F (ten days mean minimum temperature) and under 34°F .(ten days mean minimum temperature), Ixodes ricinus has hardly ever been found on foxes. The parasitism by species of the genus Pholeoixodes depends on the frequency of earth visits by foxes or on their seasonal reproductive activity, (there is a close connection between those two facts). The parasitism is very high before and during the rut, high during whelping and rearing of cubs. Three annual waves are observed. The lower number of Ph. canisuga during the intermediate periods, the sex-ratio, the proportion of larva, nympha, and adults, show by comparison with Ph. hexagonus the greater subjection to earths of the first species.