BACKGROUND: Many health behaviors have a history of repetition and, as a result, may become habitual. Because including a measure of habit strength may add depth to current theoretical models on health behavior, the present study explored the issue of habit strength within the context of the theory of planned behavior regarding adult bicycle use as a means of transportation. METHODS: Cross-sectional data were gathered in a Dutch adult sample (n=317; mean age=42.09 years; 46.7% men) in 2006 using self-administered questionnaires. Variables for the theory of planned behavior were assessed regarding bicycle use as a means of transportation; habit strength regarding bicycle use as a means of transportation was assessed with the validated Self-Reported Habit Index. Hierarchical regression analyses and interaction analyses using simple slope analyses were conducted. RESULTS: After controlling for variables for the theory of planned behavior, habit strength was the strongest predictor of bicycle use. In addition, simple slope analyses based on a significant interaction term, intention x habit, showed that intention was a significant predictor of bicycle use among those at low levels of habit strength for bicycle use (beta=0.67, p<0.001) but a weaker and nonsignificant predictor at high levels of habit strength (beta=0.10, p=0.136). CONCLUSIONS: Habit strength is a moderator of the intention-behavior relationship regarding bicycle use, with intention becoming less relevant when bicycle use increases in habit strength. Future determinant and intervention studies on physical activity may benefit from including a measure of habit strength-for instance, by identifying the differential effects of informational and environmental interventions.