The dimorphic characters of 3 different microsporidia are discussed. The dimorphic forms of 2 of these microsporidia were described as separate species. Nosema necatrix Kramer and Thelohania diazoma Kramer are 2 forms of a single microsporidium isolated from the armyworm, Pseudaletia unipuncta (Haworth). Nosema plodiae Kellen and Lindegren and Thelohania nana Kellen and Lindegren are the dimorphic forms of another single microsporidium isolated from the Indian meal moth, Plodia interpunctella (Hübner). The 3rd example of a microsporidium having dimorphic properties is an undescribed species from the sod webworm, Crambus trisectus (Walker). All 3 of these microsporidia have both a Nosema-type and a Thelohania-type developmental cycle, but at developmental temperatures of 32°C or higher, Thelohania-type development does not occur. The conditions which promote Thelohania-type development are discussed and evidence is presented that these microsporidia are single dimorphic species rather than a mixture of 2 species. The possibility that other Thelohania species described or reported from Lepidoptera may also be dimorphic species is also discussed.