LC 50 S of Steinernema carpocapsae (Weiser), S. glaseri (Steiner), and Heterorhabditis heliothidis (Khan, Brooks & Hirschmann) to third-instar Phyllophaga hirticula (Knoch) were 210, 86, and 12 nematodes per grub, respectively, when administered as peroral injections. Bioassay of the three nematodes against P. hirticula larvae in soil arenas without food, however, failed to produce discernible concentration-mortality responses. Observations indicated that, although infective nematodes remained in the soil and on the grubs 2 wk after application, a large proportion of the nematodes migrated from the soil immediately after application. Thus, the actual concentration of nematodes in the arenas was less than the concentration applied, and probably accounted for the lack of response in bioassays in soil arenas