An essential requisite for transmission of Plasmodium, the causative agent of 
malaria, is the successful completion of a complex developmental cycle in its 
mosquito vector. Of hundreds of ookinetes that form in the mosquito midgut, only 
few transform into oocysts, a loss attributed to the action of the mosquito 
immune system. However, once oocysts form, they appear to be resistant to 
mosquito defences. During oocyst development, a thick capsule forms around the 
parasite and appears to function as a protective cover. Little information is 
available about the composition of this capsule. Here we report on the 
identification and partial characterization of the first Plasmodium oocyst 
capsule protein (PbCap380). Genetic analysis indicates that the gene is 
essential and that PbCap380(-) mutant parasites form oocysts in normal numbers 
but are gradually eliminated. As a result, mosquitoes infected with PbCap380(-) 
parasites do not transmit malaria. Targeting of the oocyst capsule may provide a 
new strategy for malaria control.