The feeding rhythm of larvae of Porthetria dispar (L.) was determined from field observations. In stages I through III larvae began feeding at dawn, but not when temperatures were below 45°. The peak of feeding occurred in the morning, with some feeding observed throughout the day. When not feeding, or at night, larvae remained quiescent on the undersides of leaves. At stage IV there was a sharp shift in the feeding rhythm, with feeding initiated in the evening and reaching a peak at sunset. About midnight, feeding activity began to decline, with small increases in feeding noted before sunrise, and between 7 and 8 AM. The numbers of larvae feeding during the day were small.