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An airborne radar was used to detect insets dispersing from an area of about 200 000 hectares of maize infested with Heliothis zea (Boddie) and Spodoptera frugiperda (J. E. Smith), in the lower Rio Grande River Valley (LRGRV) of northeast Mexico. During the night of 20 June 1989, a ground-based radar, located at the downwind edge of the maize production area, detected early evening take-off and departure of flying insects. Simultaneously, an airborne radar detected low numbers of airborne insects 50 km downwind. Large numbers of flying insects were detected 200-700 m above ground as the aircraft approached the source. The edge of the emigrating cloud of insects was clearly marked by a rapid change in numbers of airborne insects; the cloud was subsequently intercepted at various locations downwind of the source area. The downwind edge of the insect cloud appeared to travel northward at least 400 km in 7.7 h. Maximum H. zea moth emergence occurred on 18 June from maize in the LRGRV. Three days later, maximum oviposition occurred in cotton near Uvalde, 370 km from the LRGRV and downwind on the night of the mass migration observed.