The relationship between the sporozoite of Eimeria papillata and the host cell was studied by comparing the frequency and distribution of intramembranous particles (IMP) in their respective plasmalemmas before and after invasion. The entering sporozoite invaginates the host cell membrane during entry and this forms the boundary of the vacuole that encloses the parasite. Freeze-fracture replicas of this parasitophorous vacuole (PV) membrane showed a 3 fold decrease in IMP on the P- face (400 ± 215) and E-face (100 ± 29) when compared to the uninfected cell membrane (1369 ± 124 on the P-face and 395 ± 79 on the E-face). Studies of other coccidia and Plasmodium suggest that this considerable modification of the membrane appears to be the result of addition of proteins and lipids which are contributed by the invading parasite. The P- and E-faces of the plasmalemma of the free or internalized sporozoite was virtually unchanged. © 1993, Gustav Fischer Verlag Jena. All rights reserved.