Dynamique saisonniere des nymphes et adultes d Ixodes ricinus en phase libre sur la vegetation, dans le foyer alsacien d encephalite a tiques. - From 1971 to 1974, 6.304 nymphs and 2.043 adult females of Ixodes ricinus were collected from vegetation in the forest, in the south of Strasbourg (Alsace). For bath stages activity begins in March; maximum of nymphal;!ctivity is in May, with a brutal decrease in July; adult activity is maximal from_)June until August. The nymphal and adult distribution in the forest seem heterogeneous; in one station, where adult host mammals are, adult ticks are much more abundant than nymphs: 46,5% compared to Jess than 21 % in other stations.