A peritrophic membrane was found in all three stages of Ixodes ricinus at no later than 18 h after their placement on rabbits. It was found to remain intact until at least 11, 30 and 10 days after repletion in larvae, nymphs and females, respectively. In blood-feeding I. ricinus, the peritrophic membrane is an uneven single layer with a thickness of about 0.03-0.48 micron in larvae, 0.03-0.79 micron in nymphs and 0.04-0.93 micron in females. It covers the whole surface of the midgut epithelium at a distance of about 0.2-0.8 micron. After repletion, the peritrophic membrane becomes thicker and thicker and more and more winding and simultaneously becomes multi-layered mainly in its arched parts. The distance between the peritrophic membrane and the midgut epithelium increases considerably and in the arched parts can reach as much as about 13 and 16 microns in metamorphosing larvae and nymphs, respectively and 25 microns in ovipositing females.