This paper presents the ultrastructure and characteristic features of Nosema leptinotarsae infecting L. decemlineata collected in Trabzon (Turkey). Fresh spores are oval, 4.69 +/- 0.19 mu m in length and 2.43 +/- 0.12 mu m in width (n = 50). The spore wall is thick and measures 180 to 250 nm and consists of a clear endospore (130 to 170 nm) and an electron-dense, uniform exospore (50 to 80 nm). The polar filament is isofilar and has 15-16 coils. The well-developed polaroplast has a lamellated structure with thin lamellae. The ultrastructural features of Nosema leptinotarsae differ from those of other microsporidia infecting chrysomelids.