Entomopathogenic nematodes were isolated from a population of larvae of the cutworm Agrotis segetum Denis & Schiffermuller, 1775 (Lepidoptera: Noctuidae) collected from the Black Sea region of Turkey in 2009 using White traps. The isolated nematodes were identified as Steinernema websteri based on morphological and molecular properties. Steinernema websteri was also found for the first time in Turkey and in A. segetum larvae. The efficacy of isolates was tested on A. segetum larvae in plastic boxes. Different concentrations of nematodes (100, 300, and 500 infective juveniles (IJs) g(-1) of dry sand) at 23 +/- 1 degrees C were used. The obtained mortality percentages were 100% with the concentration of 500 IJs g(-1) of dry sand within 5 days after application under laboratory conditions. Our results indicate that S. websteri is a promising biological control agent against A. segetum, which is one of the most serious subsoil pests plaguing agricultural crops and fruits worldwide.