Fear of physical activity (FoPA) is prevalent in patients with heart failure and associated with lower physical activity despite medical exercise prescriptions. The present study examined physiological indicators of FoPA by assessing startle modulation and heart rate responses after affective priming with lexical stimuli of positive, neutral, and negative valence, as well as words related to physical activity as potentially phobic cues. After screening for FoPA in patients with heart failure and healthy adults, twenty participants each were assigned to one of three subsamples: a healthy control group and two cardiac patient groups scoring either low or high on FoPA. The high-FoPA group showed more pronounced startle potentiation and heart rate acceleration (i.e., mobilization of defensive behavior) in the phobic prime condition compared to controls. Differences in FoPA accounted for 30% of the startle potentiation by phobic priming, whereas general anxiety, depression, and disease severity were no significant predictors in patients with heart failure. These findings suggest that heart failure-associated FoPA elicits avoidance behavior at a largely automatic level, and might thereby contribute to low adherence to exercise regimen. Thus, FoPA should be addressed in the design of psychological interventions for cardiac patients to foster physical activity.Copyright © 2020. Published by Elsevier Inc.