OBJECTIVE: Exercise videogames are increasingly popular as individuals seek to increase their activity. This study assessed the interaction of exercise self-efficacy (ESE) levels and perceived task demands of a virtual game on exercise and physiological reactions. Perceived task demands were manipulated by changing parameters of the participant's avatar in the exergame. METHODOLOGY: An experimental design was used with 89 female undergraduate students (Mean age=18.74+1.05, 18 to 21 years) who were randomly assigned to the high task demand condition (avatar weight remaining constant) or low task demand condition (avatar losing weight). Prior to the biking task, the Exercise Self-Efficacy scale was used to measure ESE. Participants biked on a stationary bike while immersed in a virtual environment with an avatar and were told the avatar would slim if they cycled fast enough. Only half of participants' avatars actually slimmed (high task demand manipulation). During biking, cardiovascular measures were assessed using a Biopac MP150 system with an Impedance Cardiograph and CNAP Monitor 500. One week after completion of the study, participants reported amount of exercise completed in the last week (in 15 minute increments) via the Physical Activity Questionnaire. Moderated regressions and moderated mediation models were used to examine the interaction of ESE levels, experimental condition and cardiovascular reactivity on amount of strenuous physical activity. RESULTS: Individuals with high ESE showed similar results in both task demand conditions with physiological reactivity patterns indicative of threat. Individuals with low ESE with a static avatar completed less exercise than those with low ESE with a slimming avatar. CONCLUSION: Results confirm that individuals with low ESE are particularly vulnerable to exercise demands.