A new microsporidian pathogen of the cabbage butterfly, Pieris brassicae is described based on light microscopy, ultrastructural characteristics and comparative small subunit rDNA analysis. The pathogen infects the gut of P. brassicae. All development stages are in direct contact with the host cell cytoplasm. Meronts are spherical or ovoid. Spherical meronts measure 3.68 +/- 0.73 x 3.32 +/- 1.09 mu m and ovoid meronts 4.04 +/- 0.74 x 2.63 +/- 0.49 mu m. Sporonts are spherical to elongate (4.52 +/- 0.48 x 2.16 +/- 0.27 mu m). Sporoblasts are elongated and measure 4.67 +/- 0.60 x 2.30 +/- 0.30 mu m in length. Fresh spores with nuclei arranged in a diplokaryon are oval and measure 5.29 +/- 0.55 mu m in length and 2.31 +/- 0.29 mu m in width. Spores stained with Giemsa's stain measure 4.21 +/- 0.50 mu m in length and 1.91 +/- 0.24 mu m in width. Spores have an isofilar polar filament with six coils. All morphological, ultrastructural and molecular features indicate that the described microsporidium belongs to the genus Nosema and confirm that it has different taxonomic characters than other microsporidia infecting Pieris spp.