Children with autism have shown substantial benefits from rigorous physical activity, however, it is often difficult to motivate these children to exercise due to their usually sedentary lifestyles. To address the problem of motivation, we have developed Astrojumper, a stereoscopic virtual reality exergame which was designed to fit the needs of children with autism. We use electromagnetic trackers and a 3-sided CAVE to present virtual space-themed stimuli to the user, who must use physical movements to avoid collisions and gain points. We can use Astrojumper not only to motivate exercise, but to evaluate the different ways people with and without autism interact with an exercise tool. Preliminary playtesting of Astrojumper has been positive, and we plan to run an extensive evaluation assessing the effectiveness of this system on children with and without autism.