Potato aphid, Macrosiphum euphorbiae (Homoptera: Aphididae), is an abundant potato pest and vector of potato leaf-roll virus and potato virus Y in Maine and other potato growing areas. We investigated the circadian rhythmicity of its movement towards host plant odor. Effects of daily cycle (day or night) and illumination (light or dark) on the proportion of aphids colonizing potato leaflets were determined in a Petri plate arena and in a Y-tube olfactometer. In Petri dishes, both daily cycle and light had a highly signifi- cant effect on plant colonization. Increasing temperature reduced aphid colonization of the leaflets. In the olfactometer, light had a significant effect on the proportion of aphids walking towards the host plant. Interaction between time and light was also statistically significant, with the effect of illumination being smaller during the day than during the night. Our results suggest that circadian rhythm in host-finding behavior of the potato aphid is regulated by both exogenous and endogenous mechanisms.