The development of yellow fever virus in mice was studied by electron microscopic examination of infected brain and liver. The different organs were also titrated for the sequential production of infective virus post-inoculation. Virions were seen only in the brain; none was observed in the liver even though both organs had good titres of infective virus. The virus titre in the brain was, however, usually about one to two logs higher. In the brain complete virions were found in vacuoles and cisternae of the endoplasmic reticulum, presumably by a budding process from the membrane. The membranes of these intracellular spaces appeared to be the main site for virus assembly. Elongated tubular structures, in close association with virus particles, were seen in some vacuoles. Most of the mitochondria were distended and the cristae were either disrupted or absent. There was no nuclear involvement at any stage. The mature virion measures about 40 nm, made up of a core of about 35 nm and an outer membrane of about 2.5 nm.