ABSTRACT Malaria remains one of the most devastating infectious diseases. It kills over a million people every year while causing immense suffering and economic losses worldwide. Whereas much progress has been made in understanding the life cycle of the parasite in the human host and in the mosquito vector, significant gaps of knowledge remain. Fertilization of malaria parasites is a poorly understood process that takes place in the lumen of the mosquito gut. This process is important because survival in nature is completely dependent on the ability of the parasite to undergo sexual reproduction. The proposed research aims to identify molecular interactions that take place during fertilization of malaria parasites. This project is based on an unorthodox approach (identification of peptides that bind to the gamete surface) made possible by the recent development of an important tool, a transgenic parasite that produces red-fluorescent female gametes and green-fluorescent male gametes. Pure populations of malaria female and male gametes from this transgenic parasite were isolated by cell sorting and then used to screen a phage display library for peptides that recognize molecules on the gamete surfaces. A peptide (FG1) that binds to female gametes and another peptide (MG1) that binds to male gametes were identified. Importantly, when added to a malaria infectious blood meal, each of these peptides blocked parasite fertilization, suggesting that the peptides bound to a receptor and prevented its interaction with a ligand on the gamete of the opposite sex. Our working hypothesis is that the peptide structurally mimics the gamete ligand and that peptide and ligand compete for binding to the corresponding receptor. This proposal lays out a research plan to identify the receptor to which the FG1 and MG1 peptides bind and the ligands on the gametes of the opposite sex that the two peptides structurally mimic. An additional aim is to characterize the proteome on the surface of female gametes before and after fertilization, to gain insights on additional proteins involved in fertilization and possible block to polyspermy. Elucidation of mechanisms of fertilization is important not only for understanding the basic biology of malaria and other parasitic diseases but could also lead to the identification of new targets for blocking transmission and the spread of disease. Moreover, should the mechanisms be conserved, our findings could be extended to the biology of fertilization of higher organisms.