The microsporidium Unikaryon phyllotretae sp. n., a new pathogen of Phyllotreta undulata, is described based on light microscopic and ultrastructural characteristics. Microscopic examination of parasitized individuals revealed two types of spores. The majority of the spores were of the first type, which are oval and measured 2.74 +/- 0.17 x 1.93 +/- 0.17 mu m when fresh. Fresh spores of the second type (very rare) are elongated and measured 4.39 +/- 0.18 x 1.61 +/- 0.20 mu m. All life stages have single nuclei. Sporogony ends with uninucleate single sporoblasts and spores. The spores were only observed in Malpighian tubules. The isofilar polar filament of the parasite has six to eight coils, and a well-developed polaroplast was of the lamellated type, with closely packed anterior lamellae and loosely packed posterior lamellae. (C) 2009 Published by Elsevier GmbH.