Little is known about the genetic structure of various species and populations of entomopathogenic nematodes. We determined genetic variability within and among isolates of seven Heterorhabditis species using random amplified polymorphic DNA (RAPD) markers. We used 10 random primers which were previously identified as useful to quantify genetic variability among these species. Mean percentage similarity among the individuals of conspecific species was 96.25%, whereas, the mean value among different isolates for three species was 83.8%. Mean percentage similarity among different species was 31. 3%. The banding patterns produced by RAPDs positively correlated with described morphological classification; however, H. hawaiiensis could not be separated from H. indicus, or H. marelatus from H. hepialius. RAPD profiles placed an unidentified isolate (IS5) with H. indicus. Copyright 1998 Academic Press.