The effect of first-generation southwestern corn borers ( Zeadiatraea grandiosella Dyar) on the development and yield of dent and sweet corn was studied at Stillwater, Okla., in 1955. Plants in different stages of growth were manually infested with newly hatched larvae. Plant height was not significantly affected, but the weight of plants was reduced moderately by the borers. There was a greater reduction in yield of grain from small than from large corn plants. Dead heart was also more prevalent in the plants that were smallest at the time of infesting. Since small plants had greater damage than larger ones, the earliest planting consistent with good agronomic practices should avoid the greatest damage by the first generation of this borer. Dissections made at harvest did not give an accurate measurement of first-generation populations.