Microsporidia are obligate fungal intracellular parasites of all animal taxa. Among them the genus Nosema (Nosematidae) is known as the most common entomopathogen. Of these parasites, the ribosomal organization is one of the most pronounced molecular characteristics. One type is the normalarrangement of small subunit (SSU)-internal transcribed spacer (ITS)-large subunit (LSU) in the DNA sequence order. The other is the reverse arrangement of LSU-ITS-SSU. The latter is assigned to be the ‘true’ Nosema in the Nosema/Vairimorpha clade. However, we found that the SSU sequence of a strain of Nosema species having the normal arrangement of its rRNA sequence seemed to be more closely related to the ‘true’ Nosemagroup. Consequently we have further analyzed the complete sequence of rRNA. The results imply that there might be arecombination event in its rRNA evolution and/or the strain may form a novel group near the ‘true’ Nosema group. Interestingly both SSU and LSU of the ‘true’ Nosema and others may be under different selection pressure. We have also found that the size of ITS is distinct between the ‘true’ Nosema and other microsporidian species within the Nosema/Vairimorpha clade. This feature should be a useful diagnostic tool to distinguish the ‘true’ Nosema from others in the clade.