An electron microscopic study was performed on the ultrastructure of the Mukai strain of Japanese encephalitis virus propagated in vitro on PS cells. PS cells were cultivated in bottles containing 10% calf serum and 0.5% lactalbumin hydrolysate in Earle's balanced solution. Titration of cell-associated virus was estimated by cytopathic effect. At various intervals following infection, the cells were fixed in buffered 1% osminum tetroxide-solution, embedded in methacrylates and cut on a Leitz ultramicrotome. After the sections were stained in saturated uranyl acetate solution, they were observed in the Hitachi type HUll electron microscope. It is suggested that the JE virus particle is hexogonal in shape and approximately 40 mu in the longest diameter, it is composed of an outer membrane 20 A° thick viroplasm 30 A° thick, and an electron dense nucleoid 25 mu in diameter.