In central and eastern Iowa, adults of the European corn borer (ECB),Ostrinia nubilalis(Hübner), were captured hourly with blacklight traps (BLT) in and away from action sites (dense vegetation where ECB adults mate and rest) for 14 nights during the second flights of 1980 and 1981. Also, samples of moths in the grass at the action sites were taken hourly. Peak capture of females in the BLT occurred immediately after dusk, whereas peak male capture occurred during the period of greatest mating activity (ca. 2400 to 0100 h). Captures of males remained much greater than those of females throughout the rest of the night, indicating that, during this period, males are captured while seeking less mobile females. This difference in mobility explains the tendency of males to outnumber females in BLT. Males tended to outnumber females in captures obtained daily in action sites with a drop-net. The drop-net data suggest that, after oviposition in cornfields, not all mated females return to the action site each night, but that males usually remain in the action sites because of the pheromone released by the unmated females.