Distribution of entomopathogenic nematodes of the families Steinernematidae and Heterorhabditidae in potato fields in north-west Iran (Nematoda: Rhabditida)Abstract : During 2009 a survey on entomopathogenic nematodes was conducted in potato fields in two provinces in the north-west of Iran. Out of a total of 320 soil samples, 9 were positive for entomopathogenic nematodes (2.81%), with 7 (2.18%) containing Heterorhabditis and 2 (0.62%) Steinernema isolates. Morphological and molecular studies were undertaken to characterize these isolates. The Heterorhabditis isolates were identified as Heterorhabditis bacteriophora and one new undescribed species and two species of Steinernema isolates were identified as S. feltiae and S. carpocapsae. Heterorhabditis bacteriophora was recovered from 6 sites and Heterorhabditis n. sp. Nas7 from only one site like two other species of Steinernema.