The effect of entomopathogenic nematodes on nontarget arthropods in the laboratory, field soils, and a stream were assessed. In the laboratory, adult predators were less susceptible to the nematodesSteinernema carpocapsae(Weiser) (Rhabditida: Steinernematidae) andHeterorhabditis bacteriophoraPoinar (Rhabditida: Heterorhabditidae) than the immature stages. In field tests, entomopathogenic nematodes that had significantly suppressed pest populations (Popillia japonicaNewman, Japanese beetle,Scapteriscus vicinusScudder, tawny mole cricket,Otiorhynchus sulcatus(F.), black vine weevil,Delia radicum(L.), cabbage maggot, andDiabrotica virgifera virgiferaLeConte, western corn rootworm) did not adversely affect the numbers of nontarget soil arthropods in comparison with the untreated control. In contrast, broad-spectrum chemical insecticides (isofenphos, ethoprop, or chlorpyrifos used as chemical checks) significantly reduced or showed a tendency to reduce nontarget arthropod populations. In a stream trial,S. carpocapsaesignificantly reduced black fly larval populations, but the nontarget insects often increased in the treatment sites. Decreases in nontarget populations were matched by approximately equal or greater reductions in the upstream controls. We conclude that entomopathogenic nematodes do not adversely affect nontarget arthropods when used for short-term control of insect pests.