Aligned 265-bp sequences of partial 18S rRNA gene were used to infer phylogenetic relationships among entomopathogenic nematodes by using maximum parsimony and likelihood methods. Phylogenetic analyses support Heterorhabditidae and Steinernematidae belonging to different monophylies. There was more sequence divergence in Steinernema species than in Heterorhabditis species. These results are congruent with the phylogenies based on morphological, life cycle, and distributional evidence. Examination of all trees within 1% of the length of the most parsimonious trees and bootstrap analyses support most relationships among Steinernema species but the relationships among Heterorhabditis species were not supported. We suggest that the partial 18S rRNA gene sequences may be too conserved for phylogenetic inference among Heterorhabditis species, but are well suited for phylogenetic inference within and among closely related families and genera of entomopathogenic nematodes and for inferring phylogenetic relationships among Steinernema species.