A series of field trials evaluated the efficacy of the entomopathogenic nematode, Steinernema carpocapsae Weiser All strain for control of western corn rootworm, Diabrotica virgifera virgifera LeConte, larvae. Separate trials examined the effects of nematode application rate and timing on corn rootworm control in 1991. In the rate trial, S. carpocapsae (0, 10⁴, 10⁵, 10⁶, and 10⁷ nematodes per 30.5 row-cm) was applied as the insect population entered the second instar. Nematode treatment significantly reduced corn root injury and adult emergence. High application rates (10⁶ and 10⁷ nematodes per 30.5 row-cm) significantly outperformed low rates (10⁴ and 10⁵ nematodes per 30.5 row-cm). Nematode treatment (500,000 S. carpocapsae per 30.5 row-cm) reduced root injury and adult emergence slightly in the timing trial; later applications (second, third instar) were numerically, but not significantly, more effective than early (egg, first instar) ones. In 1992, a factorial trial combined four application dates (egg; first, second, third instar) and five rates (0, 10⁴, 10⁵, 10⁶, 10⁷ S. carpocapsae per 30.5 row-cm). Later applications (second, and particularly, third instar) were significantly more effective than early (egg, first instar) ones. Root injury and adult emergence in control and low rate (10⁴ and 10⁵ nematodes per 30.5 row-cm) plots significantly exceeded that in high rate (10⁶ and 10⁷ nematodes per 30.5 row-cm) plots. Commercially acceptable root protection (root injury ratings below 3.0) was provided by 10⁶ nematodes per 30.5 cm of row applied to third instars, and by 10⁷ nematodes per 30.5 cm of row applied to second and third instars.