Abstract Laboratory evaluations were performed to determine pathogenicity of the entomogenous nematodes, Steinernema feltiae Filipjev (= Neoaplectana carpocapsae Weiser) and Steinernema bibionis (Bovien), to carpenterworm, Prionoxystus robiniae . Likewise, Heterorhabditis heliothidis (Khan, Brooks, and Hirschmann) was also evaluated in addition to the aforementioned species against the locust borer, Megacyllene robiniae . The carpenterworm was very susceptible to steinernematid species. Carpenterworm vulnerability to parasitism by S. bibionis decreased during the fifth and sixth instars. S. feltiae proved to be the more pathogenic species in the overall comparison with LD 50 's of 1.5 and 3.7 nematodes/larvae for S. feltiae and S. bibionis , respectively. LD 50 values of 11.2, 18.8, and 23.4 nematodes/larva obtained were for S. feltiae, S. bibionis , and H. heliothidis , respectively, against the locust borer. These LD 50 's were not significantly different from one another at the 0.05 level.