Northern corn rootworm, Diabrotica barberi Smith & Lawrence, eggs were obtained from female beetles collected in August 1985 from cornfields in Champaign, Ill., and Madison, S. Dak. Eggs were buried in soil or placed in environmental chambers that closely simulated natural soil temperature conditions and were observed for hatch during 4-5 yr. Egg diapause ranged from 1 to 4 yr for both populations. Northern Corn rootworm eggs were also obtained in August 1986 from female beetles collected from four Illinois cornfields that experienced greater rootworm damage than was expected for cornfields rotated annually with a soybean crop, and from females collected from the previously sampled Champaign field. The percentage of eggs that hatched after prolonged diapause (>1 chill period) ranged from 13.9% for eggs from northwest Illinois to 51.3% for eggs from east central Illinois. When the percentage of northern Corn rootworms with prolonged diapause in a given county was regressed on the percentage of rotational corn grown in that county, a significant positive correlation was obtained ( r = 0.89, df = 3, P = 0.04).