Previous observations on the process of invasion by P. knowlesi are briefly reviewed and new findings concerning the adhesion of parasites to erythrocytes and their intracellular passage are reported. Merozoites adhere to erythrocytes by means of a well-defined coat with the cytochemical characteristics of glycoprotein. This coat has receptors that engage in at least three distinct types of attachment. The ensuing invagination of the erythrocyte surface has two phases, the first consisting of an inward buckling of the membrane to form a vacuole and the second a rapid expansion to create the final parasitophorous vacuole inhabited by the trophic parasite. Some cytochemical evidence concerning the nature of the rhoptry complex is discussed in relation to these changes.