Indirect evidence has suggested the existence of a second chitinase gene, PgCHT2, in the avian malaria parasite Plasmodium gallinaceum. We have now identified PgCHT2 as the orthologue of the P. falciparum chitinase gene PfCHT1, a malaria transmission-blocking target. Computational phylogenetic evidence and biochemical and cell biological functional data support the hypothesis that an avian-related Plasmodium species was the ancestor of both P. falciparum and P. reichenowi, and this single lineage gave rise to another lineage of malaria parasites, including P. vivax, P. knowlesi, P. berghei, P. yoelii, and P. chabaudi. A recombinant PfCHT1/PgCHT1-neutralizing single-chain antibody significantly reduced P. falciparum and P. gallinaceum parasite transmission to mosquitoes. This single-chain antibody is the first anti-P. falciparum effector molecule to be validated for making a malaria transmission-refractory transgenic Anopheles species mosquito. P. gallinaceum is a relevant animal model that facilitates a mechanistic understanding of P. falciparum invasion of the mosquito midgut. © 2005 by the Infectious Diseases Society of America. All rights reserved.