Tests were made in environmental chambers with the progeny of Ostrinia tuibilalis (Hübner) (Lepidoptera: Pyraustidae), acquired from an area in Alabama near the 1967 southern-most distribution line (north Georgia westward) and from a locality in Georgia which appears to mark a new distribution line. With photophases and temperatures simulating a late June or mid-August at the 2 locations, 23 and 69% of the Georgia and Alabama populations, respectively, went into diapause. When borers of the 2 populations were exposed to conditions typical of August 15 in southern Georgia, the Georgia borer, but not necessarily the Alabama borer, proved to have adapted to the photophase and temperature range prevalent in midsummer along the upper Gulf Coast; however, host source and time of year of collection of borers may also affect the diapause response of borers to photophase and temperatures. Although diapause usually contributes to the abundance and dispersal of a species, it appears that during summer, until recently, the response has been working against abundance and dispersal of the European corn borer in the Gulf Coast area.