The European corn borer (ECB), Ostrinia nubilalis (Hübner), is a major pest of maize across the US Corn Belt, and is a primary target of the Cry1Ab toxin in transgenic Bt-corn. There is concern that the ECB could evolve resistance to Bt-corn because of the high selection pressure the insect has been under with widespread adoption of the transgenic technology. The effectiveness of current insect resistance management strategies, as well as mitigation strategies to be implemented if resistance is detected, depends fundamentally on understanding adult dispersal, mating, and the relative timing of the two. However, much remains unknown about these behaviors. ECB adults are commonly found concentrated in grassy aggregation sites, where they rest during the daytime and mate at night. Mark-release studies indicate that most newly-emerged ECB moths disperse beyond at least 300 m before colonizing acceptable aggregation sites. How much further they disperse remains unknown. This unexpected finding suggests the possibility of an obligatory dispersal phase of ECB adults after eclosion from the pupa. Flight mill experiments indicate that unmated females average over 15 km of flight during the first night after emergence, compared to about 5 km for 1-day-old males. Average flight distance decreased with age. Flights as long as 39 and 31 km were observed for females and males, respectively. Effects of mating and Nosema infection on flight behavior will also be reported. Implications of obligatory dispersal behavior and differential dispersal among the sexes for ECB resistance management in Bt corn will be discussed.