Out of the 9 small mammal species present in the forest, Apodemus sylvaticus, A. flavicollis and Clethrionomys glareolus are the most numerous (80 % of the total collected) and the most tick-infected ones. This parasitism, nearly exclusively Iarval - Jess than 1 % of the 9 056 collected are nymphs - is due to Ixodes ricinus. The tick-rodent contacts are significant as parasitized-rodents ratio is always more than 50 % with maxima ranging from 65 to 90 %, according to year. Larval activity varies mainly with the year. Anyway, every year the annual activity peak can be observed in May. At this period, very favourable conditions to virus circulation take place in the forest. Whereas tick - larval population grows very siginficantly, the rodent population remains low, this situation leads to a parasite concentration on the present individuals. Furthermore it is also at this very period that the rodent population rejuvenates with the arrivai of young virus-receptive cohorts.