Trissolcus japonicus (Ashmead) (Hymenoptera: Scelionidae) is an egg parasitoid wasp from East Asia. It is considered to be an important natural enemy of the brown marmorated stink bug, Halyomorpha halys (Stål) (Hemiptera: Pentatomidae). Ongoing efforts to evaluate the physiological host range of this potential classical biological control agent have demonstrated that the wasp can parasitize at least 11 genera of nontarget Pentatomoidea in North America in no-choice tests. However, factors such as host-acceptance behavior that might affect nontarget parasitism in the field are poorly understood. We evaluated the preference of T. japonicus females provided with egg clusters of H. halys, Thyanta custator accerra McAtee (Hemiptera: Pentatomidae), Podisus maculiventris (Say) (Hemiptera: Pentatomidae), and Euschistus variolarius (Palisot de Beauvois) (Hemiptera: Pentatomidae) when wasps were reared on the target pest, H. halys, and the nontarget species, T.c. accerra and P. maculiventris. When reared on H. halys, T. japonicus showed a strong preference for H. halys, and frequently rejected nontarget hosts after inspection. When T. japonicus was reared on nontarget hosts, females showed reduced host-specificity, but were significantly smaller and produced a lower number of offspring.These results suggest that innate host-acceptance behavior is likely to reduce parasitism on some nontarget Pentatomoidea which T. japonicus will encounter in the field. Future efforts should be made to examine the ability of the wasp to reproduce long term on nontarget species when H. halys eggs are limited.