Stability balls have become a common option for alternative seating in classrooms. While the social validity of stability ball classroom seating remains high among students and teachers, findings on the effectiveness of this alternative seating method are mixed. This is particularly true when the intent of the intervention is to increase student on-task behavior. This study examined the effects of stability ball seating on students' on-task behavior in a general education setting, using a within-subjects repeated measures design. Researchers observed twenty-four third-grade students in a public-school classroom across three conditions: all chairs, all balls, or choice. Across observation days, the study controlled for time of day, teacher, and classroom environment. Results indicate students are on-task less frequently when sitting on a stability ball. Student reports on social validity replicated prior findings; students found the stability ball to be desirable and easy to use.