The microsporidian genus Nosema is characterized by development in direct control with host cell cytoplasm, diplokaryotic
nuclei throughout development and disporous sporogony. The genus Vairimorpha exhibits the same features plus an
octoporous sporogony producing uninucleate spores in a sporophorous vesicle. A microsporidium from diamondback
moth, Plutella xylostella, falls between Nosema and Vairimorpha in that it initiates but fails to complete the octosporous
sequence in this host. The name Vairimorpha imperfecta n.sp. is proposed. Merogony is mainly by formation of buds from
multinucleate meronts, the buds remaining attached in chains. Diplokaryotic spores measure 4·3×2·0 μm (fresh) and
have 15·5 coils of the polar tube in 1 rank. The octosporous sporogony is aborted owing to irregular formation of nuclear
spindles, incomplete cytoplasmic fission and bizarre deposition of electron-dense episporontal secretions. Phylogenetic
analyses of the sequences of the small subunit rRNA genes of V. imperfecta and of several Nosema and Vairimorpha spp.
place V. imperfecta in a clade with Nosema spp. from Lepidoptera rather than in the clade containing the more typical
species of Vairimorpha. It is suggested that the ancestors of the Vairimorpha/Nosema complex of species exhibited both
disporous and octosporous sporogonies, as does the type species of Vairimorpha, Vairimorpha necatrix. It would follow
that true Nosema spp. have lost the ability to express an octosporous sequence and that V. imperfecta is in the process of
losing it. It is proposed that the genera Nosema and Vairimorpha be placed in the same family Nosematidae Labbé 1899,
rather than in separate families and orders as at present.