The virulence of new nine heterorhabditid isolates from South Carolina (Heterorhabditis megidis LEX, Heterorhabditis zealandica EDS and CHR, and Heterorhabditis bacteriophora WPS, SMP, PD, CFG, MF and CFM strains) on the yellow mealworm was compared with two known heterorhabditid nematodes (H. bacteriophora Hb and HP88 strains) under laboratory conditions. The Petri-plate bioassay procedure was used to evaluate the susceptibility of the Tenebrio molitor larvae to the heterorhabditids at concentrations of 5, 10, 25, and 100 infective juveniles (IJs) per larva. Mortalities were counted for 4 days. At the final count, mortalities were 27.1-79.9, 48.7-85.2, 82-100, and 93.8-100% for all nematode strains at the concentrations of 5, 10, 25, and 100 IJs per larva, respectively. H. megidis LEX strain differed than others by having 100% mortality in both 25 and 100 nematode concentrations. It had also the highest mortality rate with 80% at 5 nematodes per larva and H. bacteriophora HP88, WPS and SMP strains followed it with 70.1, 70.1 and 64.6% mortality, respectively. LC50 value for the nematodes was relatively low ranging from 1.74 IJs per larva for H. bacteriophora WPS strain to 11.12 IJs per larva for H. bacteriophora Hb strain. The LT50 value ranged from 1.30 to 5.31 days. Our results suggest that H. bacteriophora WPS, SMP, and CFM, and H. megidis LEX strains may be considered first to be studied further as potential biocontrol agents of insects.