Fruit flies (Drosophila melanogaster) are a widely studied model species for addressing basic and applied biological questions, including the obesity epidemic (Hardy, Physiology and genetics of starvation-selected $$ backslashhbar $$. Las Vegas: Digital Scholarship@UNLV, 2016). Biologists investigate the effects of obesity through studying the physiology and behavior of normal and obese flies in response to exercise, diet, and other experimental conditions. In this paper, we propose an instrument, called ``Fly Roller’’, for exercising flies in biological experiments with or without measurement of metabolic rate. Fly Roller comprises two parts: a roller mechanism and a controller circuit. The roller mechanism supports and slowly rotates a plastic tube containing fruit flies, which reflexively walk along the inner wall of the tube. When metabolic measurements are desired, a gas analyzer can be coupled to a modified tube design with air valves at the ends of the tube, allowing the air lines to remain stationary in low-friction bearings while the tube containing the flies is rotated.