In order to investigate some properties of a small flacherie virus (SFV) of the silkworm, Bombyx mori L., the virus was isolated and purified from a mixed virus suspension of SFV and a flacherie virus (FV). A light radioautographic studies of larval midgut infected with SFV revealed that DNA synthesis occurred predominantly in the infected nucleus of columnar cell during the SFV multiplication, whereas no essential difference in the pattern of RNA synthesis was demonstrated between healthy and diseased midguts. By means of polyacrylamide-gel electrophoresis a comparison of polypeptides of structural viral proteins was made. The result indicated that SFV protein was composed of two polypeptides with molecular weight of about 49, 200 and 73, 500, while FV protein consisted of one polypeptide with molecular weight of about 35, 000. Immunoelectrophoretic studies of SFV, FV and a small flacherie virus strain (Ina strain) revealed that SFV was serologically different from FV, but was all the same to Ina strain.