This study presents concurrent measurements of the vertical profiles of aphid density and airstream temperature within the planetary boundary layer over east-central Illinois for 16 collection periods during a variety of synoptic weather conditions in the 1984 and 1985 growing seasons. Aphid density was determined at a fine altitudinal resolution between the ground surface and an altitude of ≈1,000 m using two helicopter-mounted collectors. These collectors allowed sampling of insects at>20 altitudes in a single flight, captured insects in a condition that permitted easy identification, and permitted accurate calculations of air volume and insect density. Vertical distribution of aphids on individual collection days was interpreted with respect to lapse rate of air temperature. The results of this study indicate that inversion layers produce stratification of aphids in the atmosphere and that aphids migrating into the Midwest are concentrated in zones within the planetary boundary layer.