The spermatophores in mated female European corn borers, Ostrinia nubilalis (Hübner), were shown to lose volume (depletion), change colour, and retract their spiral stalks over a period of about 7 days in such a way that postmating age could be estimated from their appearance. Colour changes and stalk retraction occurred more slowly in females that laid fertile eggs than in those that did not, but depletion occurred at the same rate. The average postmating age was estimated as 5.2, 6.3, 3.9, and 2.6 days for small light trap catches of first generation females in 1974, 1975, 1976, and 1977 and 4.4, 3.9, and 3.2 days for larger catches of second generation females in 1974, 1975, and 1977. Catches did not show decreasing numbers in successive age groups but tended to show modal values at < 1 day and at 4–6 days with only 1% living > 7 days. Redistribution of the moths from the date of capture back to the date of mating tended to give a unimodal curve of numbers on time in the 1975 second generation but not in 1974 or 1977. The unimodal curve of redistributed moths in 1975 was significantly different from a normal distribution. The redistribution showed that 58% of the moths mated within the heat unit interval when the second generation was expected to emerge, whereas only 38% of the actual catches occurred in this interval. Redistribution also showed that mating probably occurred even on cold nights when few moths were trapped, and conversely that on some warm nights with large catches most of the moths had mated on earlier nights.