Reproductive development of female European corn borer, Ostrinia nubilalis (Hübner), was investigated and a classification system proposed. Females collected in a blacklight trap during 1982 and 1983 were dissected and their reproductive system examined. Female reproductive systems were divided into six stages based on ovum development within the ovarioles, ovum depletion, ovariole appearance, and fat body color and shape. The female reproductive systems were also staged on the basis of spermatophore appearance. The time necessary to classify a female is also reported. Based on the classification system, the relationship between female age and stage of ovarian development was quantified under three temperature regimes. Females were found to experience a 3- to 5-d preoviposition period before initiation of egg deposition under optimal temperature conditions. This delay between adult emergence and initiation of egg laying corresponded with more advanced ovarian developmental stages collected in blacklight traps and indicates that actively ovipositing females are primarily being collected in blacklight traps.