In July, 1952, during studies on the Colorado potato beetle, Leptinotarsa decemlineata (Say), at Merivale, Ontario, a large-scale dispersal of the larvae was observed. A field of early potatoes consisting of seven 100-foot rows, which was to be used as a source of supply of the insect for plant resistance studies, had been artifically infested on June 30 by placing an overwintered adult on each plant. The beetles laid eggs in largc numbers and by the second week of July the plants were overpopulated wit11 larvae. By July 14 the plants were stripped of foliage and the larvae were forced to feed on the stalks. On the morning of July 16 the starving larvae began to leave the plants in large numbers. Almost all left on the one day, some of them travelling considerable distances.