The black cutworm,Agrotis ipsilon(Hufnagel), is sometimes a highly destructive pest of a large number of seedling crops (e. g., clover, soybeans, tobacco)throughout the United States. The factors contributing to these occasional economically important infestations are not entirely understood, especially in the Midwest. In Tennessee, Crumb (1929), in the Central Great Plains, Walkden (1950), and in Wisconsin, Apple (1967) concluded thatA. ipsilonoverwinters in the pupal stage, since they could not carry the larval stage through the winter. Other workers apparently assumed that infestations in cultivated crops arose from springemerging adults, though Davidson and Peairs (1966) did indicate overwintering in the insect's larval stage. However, they did not specify any geographical area, and they may have had reference to subtropical areas where infestations are known to occur during the winter (Oatman and Platner 1972). In the present paper we present evidence that infestations ofA. ipsilonin the Midwest result from overwintering larvae. Our evidence is based primarily on the parasitic relationship between the mermithid nematode,Hexamermis arvalisPoinar and Gyrisco, andA. ipsilon, as well as between other parasites of the pest.