Abstract
A few female moths of the silkworm were found infected with a Pleistophora like microsporidian in the spring of 1970 in Chiba prefecture. Generally, incidence of the disease was extremely low in the spring rearing season and slightly increased in the fall. The parasite was new to the silkworm, and invaded into muscles, fat bodies, Malpighian tubes and silkglands of the larvae. Pansporoblasts were observed in the infected cells, and later, sixteen or more spores were formed in a cyst. Spores were oval in shape (5.06×2.97μm) and a polar filament was about 140μm in length.
