Invasion of erythrocytes by malaria merozoites involves attachment of the merozoite membrane at the point of collision with the erythrocytes, then reorientation of the merozoite such that its apex is opposed to the erythrocyte membrane, followed by invaginatin of the erythrocyte membrane and interiorization of the parasite. Specific recognition of erythrocyte surface components by the merozoite has been shown by studies on the specificity of merozoites of different malaria species for a limited range of host-species erythrocytes, for erythrocytes of particular maturity, and for erythrocytes possessing particular blood-group determinants. Different malaria species capable of invading erythrocytes of the same host also exhibit differences in specificity for components on enzyme-treated erythrocytes. The attachment phase of merozoite invasion has been isolated from subsequent steps by treatment of merozoites with cytochalasin B -- they then attach to but do not invade susceptible erythrocytes. There is now evidence for other differences between initial attachment steps and subsequent invasion steps form studies on merozoite treatments in vitro which affect invasion but not attachment. It has also been shown that addition of N-acetyl-D-glucosamine to cultures of Plasmodium falciparum inhibits merozoite invasion. Elucidation of the sequence and nature of molecular interactions of merozoite and erythrocyte membrane molecules during invasion will be based on the fundamental ultrastructural observations and on the specificity of attachment and invasion steps already described.