Plasmodium vivax and Plasmodium knowlesi merozoites invade human erythrocytes that express Duffy blood group surface determinants. A soluble parasite protein of 135 kd binds specifically to a human Duffy antigen. Using antisera affinity purified on the 135 kd protein, we cloned a gene that encodes a member of a P. knowlesi family of erythrocyte binding proteins. The gene is a member of a family that includes three homologous genes located on separate chromosomes. Two genes are expressed as major membrane-bound products that give rise to soluble erythrocyte binding proteins: the 135 kd Duffy binding protein and a 138 kd protein that binds only rhesus erythrocytes. These different erythrocyte binding specificities may result from sequence divergence of the homologous genes. The Duffy receptor family is localized in micronemes, an organelle found in all organisms of the phylum Apicomplexa.