The distribution of animals is often limited or isolated into populations. Whenever a population has a boundary, there exist fringes. Five major types of population fringes—climatic, topographic, biotic, advancing, and artificial—are briefly discussed. The Minnesota and South Dakota populations of the European corn borer show a climatic fringe in the north, and an advancing and a topographic fringes in the west. The characteristics of the three types of fringe populations are described, and the practical difficulties in insecticidal control of this insect that resulted from the fringe characteristics are considered. More knowledge on fringe populations is needed and would be valuable in practical fields such as game and fishery biology and biological and chemical control of insects. It also may elucidate the basic ecological principles involved in the natural control of animal populations, and in the adaptive variation and hybridization among animal populations.