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 a Y-tube olfactometer. Dark and light conditions were created by switching on/off the lights in a small dark room. Since aphids have poor visibility in red spectrum light wavelength of above 600 nm aphids were handled in the dark using a dim red light bulb. Both daily cycle and light showed a highly significant effect on plant colonization. Their interaction was also statistically significant with the effect of darkness being smaller during the day than during the night. The largest proportion of aphids colonized potato leaflets during the day under the light conditions, and the smallest proportion colonized during the night under the dark conditions.