Ecologie et ethologie d Ixodes ricinus Linne 1758, en Suisse (Acarina, lxodoidea). Premiere note: fluctuations numeriques. - Two methods of sampling the ectoparasite Ixodes ricinus have been applied since October 1971 to a population of a lowland wood. The first method (flagging) furnishes an estimate of the number of ticks (larvae, nymphs and adults) during the free-living stage on the ground and on the vegetation. The second method consists in removing the ticks attached to the small mammals. In order to compare the population dynamics of these ectoparasites and of their hasts we have also established a monthly estimate of the numbers of small mammals. The results obtained after the first year are presented and discussed.