Objectives: Habit has been an undervalued concept in the behavioral sciences during the past few decades. One reason may be that habit has been equated with behavioral frequency. This leaves out an important characteristic of habits, i.e., the fact that repeated behavior may acquire a degree of automaticity. The present study aimed to demonstrate that exercising habit can be reliably measured, can empirically be distinguished from past frequency of exercising, and can thus be adopted as a meaningful criterion. Design and methods: A longitudinal study was conducted with two measurements one month apart among 111 students. Intentions to exercise, perceived behavioral control of exercising, past exercising frequency, and exercising habit were assessed at both measurements through an internet-based questionnaire. Exercising habit was assessed by the Self-Report Habit Index [Verplanken & Orbell (2003). Reflections on past behaviour: A self-report index of habit strength. Journal of Applied Social Psychology, 33, 1313-1330]), which breaks down the habit concept in the subjective experience of repetition and automaticity. Results: The results showed that exercising habit can be reliably measured, is stable over time, and can be distinguished from mere exercising frequency. Conclusions: In addition to frequency of behavior, measuring habit provides information about the way behavior is executed. An important element of exercising behavior is the decision to go exercise. It is argued that the habit concept is therefore particularly relevant for the initiation of and adherence to exercising. Implications of distinguishing behavioral frequency and habit for interventions are discussed. (C) 2007 Elsevier Ltd. All rights reserved.