Movement patterns of japanese beetle,Popillia japonicaNewman, and European chafer,Rhizotrogus (Amphimallon) majalis(Razoumowsky), grubs as influenced by gravity, host plant position, and external disturbances were studied in laboratory soil–turfgrass microcosms. Second through third instars just before pupation were monitored using radiographic techniques. Neonates were monitored using destructive sampling. Results demonstrate significantly different movement patterns between species and among age groups. The development stage of the grub had a large effect on Japanese beetle grub behavior and a measurable, but lesser effect, on European chafers. All larval stages of European chafers and all larval stages of Japanese beetle, except neonates and postoverwintering third instars, displayed a downward movement in response to disturbance. Neonate Japanese beetles showed little movement while postoverwintering. Japanese beetles moved upward when disturbed. European chafer grubs of all age classes displayed random vertical movement with some arrestment in or near sod. Preoverwintering and postoverwintering third-instar chafers showed less dramatic arrestment behavior than other instars tested. Second-instar Japanese beetles behaved similarly to European chafers; however, third instars behaved very differently. All third instars except those tested in late winter and early spring showed some innate downward movement in the soil microcosms. Japanese beetles tested in late winter displayed random movement with some arrestment in sod, whereas those tested in early spring exhibited upward movement and arrestment in sod.