A method for the germ-free cultivation of the mosquitoes Aedes aegypti and Aedes triseriatus was developed, and primary tissue cultures were prepared from minced larvae of both insect species. The Trinidad and the 9t strains of Venezuelan equine encephalomyelitis (VEE) virus and the Louisiana strain of Eastern equine encephalomyelitis (EEE) virus were grown in larval tissue culture of A. aegypti. The Trinidad strain of VEE virus was also grown in A. triseriatus larval tissue cultures. The growth of VEE virus in A. aegypti larval tissue culture was influenced by the length of time, the temperature, and the virus concentration used for the adsorption process, and the temperature, pH, and agitation of cultures during growth. In these cultures, the Trinidad strain grew somewhat better than the 9t strain; its latent period was shorter, its growth rate was faster, and it reached higher maximum titers of virus. However, EEE virus was superior to the Trinidad strain in each of these characteristics of growth. Some evidence suggested that a virus-inactivating material was present in larval tissue cultures of both species of mosquito. Ten serial passages of the Trinidad strain or five serial passages of the 9t strain in A. aegypti larval tissue cultures caused no detectable changes in either the virulence for mice or the distribution of plaque size of these virus strains.