The long-term viability of corn rootworm (CRW, Diabrotica spp.) resistant, transgenic corn as a single CRW management tactic may depend, in part, on appropriate weed management. Alternate CRW larval hosts (grasses) may offer refuge, decoy or delay infestation of corn. In 2001 and 2003, field trials evaluated the effects of grass density on CRW population dynamics in, and feeding injury to, MON863 (a moderate dose event) and its susceptible isoline. This randomized complete block study used a factorial combination of corn (none, isoline, MON863) and green foxtail (0, 7.5, 15 plants/sq. meter) treatments. Under moderate CRW pressure in 2001 (17 beetles/plant, isoline-only), MON863 reduced survival more than 80% and delayed median western corn rootworm (WCR) emergence by 12 days. While corn/light foxtail produced slightly fewer beetles than corn alone, heavy foxtail fostered 7 and 25% increases in isoline and MON863 emergence, respectively. Grass did not affect WCR phenology in the isoline. However, heavy foxtail accelerated emergence from MON863; WCR reached 50% emergence 10 days after their isoline counterparts. (2003 beetles are being evaluated.) In both years, MON863 plants suffered significantly less root injury than isoline plants; over 70% fewer roots were pruned to 8 or 4 cm. Less root injury was observed in isoline/foxtail than isoline-only treatments, but a slight increase was noted in MON863/foxtail over MON863-only treatments. A new finding in 2003 was the regular occurrence of internode feeding on the stalk, at nodes attacked by older CRW larvae, in corn/foxtail plots, particularly MON863/foxtail plots.