Culex tritaeniorhynchus and C. pipiens mosquitoes were infected with Japanese encephalitis virus either by intrathoracic injection or by membrane feeding. The virus maturation sites and the process of virus particle concentration in salivary gland cells were studied by electron microscopy. Occurrence of mature virions was primarily associated with intracytoplasmic viral matrices which were extraordinarily large and had a perinuclear location in c. pipiens mosquitoes. The other sign of virus replication was the proliferation of small spherical vesicles throughout the cytoplasm. It appeared that mature virions were entrapped in intracellular vacuoles and later released into the apical cavity of salivary gland cells through the fusion of these vacuoles with the apical plasma membrane. This process seemed to be associated with primary resynthesis of saliva in mosquitoes following blood feeding activity. Another type of shedding involved virus particles either singly or in mass being released directly through the apical plasma membrane. All of these events occurred only in cells of the lateral lobes of the salivary glands, which fact was confirmed by immunofluorescent staining of infected glands. The median lobe of mosquito salivary glands may have a minor or no role in the transmission of Japanese encephalitis virus.