Stability balls are promoted as an effective alternative to chairs at a classwide level, yet there are no published studies documenting classwide outcomes. Therefore, the purpose of this study was to investigate classwide effects of stability balls and attempt to provide empirical support for their use. Using an A-B-A-B reversal design, this study examined the effectiveness of stability balls in comparison to classroom chairs in a second-grade classroom. Student on-task and out-of-seat behaviors were measured using direct observation. Academic productivity was measured using curriculum-based measures of written expression. Similar patterns of responding for on-task and out-of-seat behavior were found while students were seated on traditional chairs and stability balls. Results demonstrated similar rates of improvement in writing fluency over the course of the study. Overall, teacher and student social validity measures indicated high levels of acceptability of stability balls in the classroom.