Electron micrographs of dengue viruses of Hawaiian strain and Mochizuki strain, which were adapted to mouse brain and chick embryo have been taken using purified virus materials.Purification methods employed were as follows: (I) Combination of partial purification of the virus adsorption to and elution from erythrocytes and ultracentrifuge, (2) Combination of protamine sulfate precipitation and ultracentrifuge. Studies by electron microscope of purified preparations from infected mouse brain and chick embryo showed that the infective particles of dengue virus were spherical, uniform and aggregate, average 18±2mμ in diameter, and they were considered to be the virus.The smaller particles of about 15mμ in diameter seen in infected mouse brain seemed to be normal components because the similarsized particles were observed also in the normal mouse brain.