A new microsporidium, Nosema sp. PM-1, was first isolated from Papilio machaon Linnaeus. The spore shape of the PM-1 isolate was a long oval with an average size of 3.22 μm × 1.96 μm. Ultrastructure observation showed that PM-1 had a typical Nosema common diplokaryotic nuclei structure with 10-13 polar filament coils, spore wall, plasma membrane, and anchoring disk. The complete rRNA gene sequences were obtained by polymerase chain reaction amplification and each rRNA unit was arrayed as follows: 5'-LSU (2497 bp)-ITS (179 bp)-SSU (1232 bp)-IGS (278 bp)-5S (115 bp)-3', which was the same as typical Nosema. The phylogenetic trees of rRNA, DNA-directed RNA polymerase II subunit, and tubulin genes all show that PM-1 was a sister to the clade comprising Nosema bombycis, Nosema spodopterae, and Nosema sp. PX1. The spore morphology, ultrastructure, and complete rRNA structure indicate that this isolate assigned to the ˝true˝ Nosema group, can parasitized in Papilio machaon Linnaeus, which provides a wider host range for Nosema.