The European sunn pest, Eurygaster maura (L.) (Heteroptera: Scutelleridae), is an insect of great importance to wheat (Triticum spp.; Poales: Poaceae), and other small grains in the Central Anatolia Region of Turkey. Natural parasitism of this pest by a parasitic nematode, Hexamermis sp. (Nematoda: Mermithidae), was reported for the first time in Ankara, Turkey, in 2013. During the next year, adults of E. maura were collected from an overwintering area under plant leaf litter and brought to the laboratory and sexed. After killing, they were dissected to determine the presence or absence of mermithids. Natural parasitism rates caused by the parasitic nematode were calculated individually for females and males of the sunn pest. The parasitism rates in the overwintering area were 20.0% for females and 31.6% for males during 2014. These findings suggest that Hexamermis sp. has potential as a biological control agent of E. maura.