In the US, less than 5% of adults obtain the recommended 150 minutes of physical activity (PA) per week, contributing to the onset of preventable chronic disease. Wearable fitness monitors are increasingly popular, with the goal of increasing PA levels; however, usage trends show that 75% of users stop wearing the devices after 1 month, limiting their potential benefits. Increasing self-efficacy (SE) (one’s belief in their ability to succeed at a specific task) and developing habits (automatic behaviors that occur in response to environmental cues) related to usage could improve engagement with monitors and further promote changes in PA. PURPOSE: Our purpose was to determine if change in SE and habit formation predict PA levels over a 3-month intervention using a wearable fitness monitor. METHODS: Ninety-four healthy adults (52% female; age 41.6 ± 18.4) were randomly assigned to receive a commercial fitness monitor alone or in combination with motivational interviewing and education on successful development of habits. Prior to receiving the monitors, participants completed the Self-Efficacy and Exercise Habit Survey and wore ActiGraph GT3X+ and activPAL3 accelerometers for 7 days to assess baseline physical activity levels. One week later, they were introduced to and provided with a fitness monitor to utilize at their discretion for three months. Participants returned for a follow up visit three months later, completing all baseline measures as well as the Self-Reported Habit Index (SRHI). The SRHI measured HS for wearing the monitor, checking data on the monitor and using the computer software and mobile app. A linear regression analysis was performed to assess the influence of change in SE and HS on PA levels at follow-up. Age, gender, group, and baseline PA levels were also included as predictors in this model. RESULTS: The overall model was significant (F(7, 67)=5.681, p<0.001). Significant predictors were change in SE (β=0.278, p=0.039), HS (β=0.296, p=0.009), and age (β=0.247, p=0.018). CONCLUSION: Our results demonstrate that improving SE and HS may be key contributors to success when using fitness monitors for promoting PA. Interventions utilizing fitness monitors may benefit by including components to improve these constructs.