The virulence of 14 Steinernema feltiae isolates, one S. carpocapsae and three Heterorhabditis bacteriophora isolates was compared in the laboratory against larvae, pupae and adults of the flatheaded root borer, Capnodis tenebrionis. Larval mortality ranged from 50 to 100% and pupae mortality from 0 to 70%. Adults were exposed to different nematode concentrations, 50 and 100 infective juveniles/cm2. Steinernema feltiae caused the highest mortality at the two doses tested, reaching 66.7% and 100%, respectively. Significant differences in mortality between C. tenebrionis males and females were observed. The differences in virulence observed at each stage of C. tenebrionis vary within a species and can be higher than differences among species. The results of this study suggest that some of the isolate strains have potential for improved biological control of C. tenebrionis.