The European corn borer first appeared in the southeast corner of Minnesota in 1943. It spread northward throughout Minnesota except the extreme north and northeast, and westward through corn-growing areas in South Dakota. This report reviews the history and some biological characteristics of the dispersion of the insect in the 2 States from 1945 to 1970.There was a high population build-up, or a “population swell” just behind the front of distribution. This phenomenon was shown by populations differing at only a subspecies level. On the basis of the population swells which occurred during the 26-year period, it is suggested that after the initial invasion in 1943, distinctly different populations could have invaded Minnesota in 1952 and again in 1966.