This study examines the effectiveness of two modes of exercise on self-efficacy (SE) in adolescents with Down syndrome (DS). Thirty-nine participants were randomly divided into a voluntary cycling group (VC) (i.e., self-selected cadence), an assisted cycling group (ACT) (i.e., at least 30% faster than self-selected cadence accomplished by a motor), or a no exercise group (NC). In each cycling intervention the participant completed 30 minute cycling sessions, three times per week for a total of eight weeks. Two subsets of the Physical Activity and Self Efficacy Survey were administered prior to cycling (i.e., pretest) and after the eight week intervention (i.e., post-test). The results were consistent with the hypothesis that self-efficacy would improve after ACT, however there was not improvement after the VC condition as hypothesized. It was also hypothesized that exercise perception would improve following the ACT intervention; execise perception showed a trend of improvement after ACT, but the data did not reach significance. Limitations include the wide variability of the DS population. This limitation is responsible for the variation in mental age seen in the intervention groups and could be responsible for the non-significance of the exercise perception data. To generalize our results for parents, therapists, teachers, etc., our recommendation is for persons with DS to participate in physical activity that is easy for them at first \u2014 a simplified sport or active game, assisted cycling, brisk walking \u2014 so that they have a positive experience with exercise. Showing individuals with DS that they can be proficient exercisers will likely improve their self-efficacy and motivate them to engage in more PA over time. In conclusion, eight weeks of moderate ACT exercise demonstrated a significant trend for improved self-efficacy in adolescents with DS.