Seasonal changes of populations of the European corn borer,Ostrinia nubilalis(Hübner) were observed in one early-planted corn field in Waseca, Minnesota, from 1948 to 1970. The results of the 1st 10 years, 1948 to 1957, were published in 1959. This paper presents the results from 1958 to 1970, and discusses the population changes for the entire 23-year period.During this period some agricultural practices, natural enemies, and the varied climatic factors have been responsible for the haphazard variations of corn borer populations from year to year.Superimposed on these changes, there was a long term cooling trend from 1960 to 1969. The temperature in 1970 showed an upturn that was responsible for the highest corn borer population since 1949–52 peaks. It is concluded that corn borer populations in the area were kept at relatively low levels by environmental factors, but can return to an economically significant level when temperature becomes favorable.