Life-cycle stages of the microsporidia Nosema bombycis, the pathogen causing silkworm pebrine, were separated and purified by an improved method of Percoll-gradient centrifugation. Soluble protein fractions of late sporoblasts (spore precursor cells) and mature spores were analysed by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). Protein spots were recovered from gels and analysed by mass spectrometry. The most abundant differential protein spot was identified by database search to be a hypothetical spore wall protein. Using immunoelectron microscopy, we demonstrated that HSWP5 is localized to the exospore of mature spores and renamed it as spore wall protein 5 (NbSWP5). Further spore phagocytosis assays indicated that NbSWP5 can protect spores from phagocytic uptake by cultured insect cells. This spore wall protein may function both for structural integrity and in modulating host cell invasion.