Recent studies on the motility of coccidian sporozoites have demonstrated a membrane-associated contractile system capable of moving certain intramembraneous components down the parasite surface propelling it forwards. The properties of this system resemble recorded observations on host cell invasion. In this study the invasive behaviour of Eimeria tenella and E. acervulina has been examined, with reference to the above findings, by light microscope and scanning and transmission electron microscopes. Known inhibitors of motility prevent invasion, though attachment appears unaffected. Invasion itself consists of 3 phases; attachment and orientation, induction of a parasitophorous vacuole and translocation of the parasite into the vacuole. Ultrastructural examination reveals a close membrane/membrane association maintained throughout invasion. From these results it is suggested that the parasite enters the parasitophorous vacuole by 'capping' the host/parasite junction down its body, so locomoting into the host cell. Such a model has two main advantages; it requires no additional modifications to either cell, and the specificity of membrane receptors would enable the one membrane-associated contractile system to be responsible for locomotion, antibody capping and host cell invasion.