Ecologie et ethologie d Ixodes ricinus L. en Suisse (Ixodoidea; Ixodidae). Cinquieme note: mise en evidence d une pheromone sexuelle chez Ixodes ricinus. The author shows the existence of a sex pheromone for the first time in a Prostriata, Ixodes ricinus. This substance is produced by the females and attracts strongly the males. The females are also attractecl, but not as strongly. The substance is soluble in aqueous solvents. The sensory receptors of the males are located in the Hallers organs.