This experiment examined the effects of a possible selves intervention on self-regulatory efficacy and exercise behavior among 19 men and 61 women (M age = 21.43 years, SD = 3.28) who reported exercising fewer than 3 times per week. Participants were randomly assigned to a control condition, a hoped-for possible selves intervention condition, or a feared possible selves intervention condition. The hoped-for and feared possible selves interventions required participants to imagine themselves in the future as either healthy, regular exercisers or as unhealthy, inactive individuals, respectively. Participants in the control condition completed a quiz about physical activity. Measures of self-regulatory efficacy (scheduling, planning, goal setting, and barrier self-efficacy) were taken immediately before and after the intervention. Participants who received either possible selves intervention reported greater exercise behavior 4 weeks and 8 weeks postintervention than participants in the control group. Planning self-efficacy partially mediated the effects of the possible selves intervention on exercise behavior over the first 4 weeks of the study. These findings highlight the effectiveness of possible selves interventions for increasing exercise behavior and the role of self-regulatory processes for explaining such effects.