A microsporidian parasite infecting Chaetocnema tibialis is reported for the first time and described by light and electron microscopy in the present study. The infection rate in the population at Samsun (Turkey) reaches up to 42%. Spores are oval, measure 3.52 +/- 0.41 mum (2.85-4.27) in length and 2.09 +/- 0.26 mum (1.90-2.85) in width, possess a diplokaryon and have 13 coils of the polar filament. The morphological and ultrastructural features indicate that the described microsporidian is a member of the genus Nosema. It is described as Nosema chaetocnemae sp. n. after the name of the genus of its host.