An ecological study of the European corn borer (ECB),Ostrinia nubilalis(Hbn.), was conducted in Boone County, Iowa, from 1950 through 1965. The average date of the beginning of first-generation pupation was 12 May, with completion 8 June. Moth flight usually began in late May, peaked during the 2nd and 3rd weeks of June, and ended in early July. First-generation oviposition was greatest during the last half of June. Most (69.0%) first-generation ECB eggs hatched; a significant number (25.5%) were lost, probably due to heavy rains and violent winds. Greatest total numbers of first-generation ECB egg masses and larvae occurred in maize planted 6–10 d after the first planting date. The average date of the beginning of second-generation pupation was 16 July, with the average completion 14 August. Peak second-generation moth flight was during the first half of August. Peak oviposition was in early to mid-August. Percentage of second-generation eggs missing was less than that of the first generation, probably because severe storms were less frequent during August than in June. Heavy second-generation infestations often occurred in early-planted maize; more than 65% of second-generation larvae were found in maize planted during the first 10 d of the planting period. Heaviest infestations were in maize planted 21–30 d after the first planting date. A small third-generation moth flight occurred in some years. Predators were much more numerous during the second-generation oviposition period than during the first-generation oviposition. More second-generation eggs were eaten by predators than were first-generation eggs. The most common parasitoid of the ECB wasLydella thompsoniHerting; this species was uncommon after 1959. Other parasitoids were recorded but were not numerous.