Background: Being physically active implies a process of habit formation, characterized by increased automaticity (Gardner, 2012). As physical activity (PA) is performed in consistent situations, the behavior is based on some or all of the following features of automaticity: efficiency, lack of awareness, unintentionally and uncontrollability (Bargh, 1994). The patterns of automaticity evolution could be attributable to the nature of the behavior (Lally et al., 2010) and individual dispositions (e.g., personality). In this study, the development of automatic properties of a newly initiated PA was examined and characterized according to the impulsivity of participants and the characteristics of the PA performed.Method: Participants (n=146) filled out a questionnaire about their personality (BFI-Fr), the nature and frequency of the PA they intended to practice. Then, they completed once a week during 10 weeks the Generic Multidimensional Automaticity Scale (Boiché et al. 2016), measuring automatic properties of PA. Multilevel growth models estimated the evolution of automaticity -and its components- as well as the moderating role of impulsivity and PA characteristics. We hypothesized that automaticity would increase more rapidly in participants (1) with high levels of impulsivity, (2) who performed more frequent and (3) less complex forms of PA.Results: The analyses showed a linear increase of the overall level of automaticity and lack of control over time, but no increase of unintentionality and efficiency. The frequency of PA significantly and positively predicted intercepts of automaticity and all of its dimensions. The complexity of PA significantly and negatively predicted intercepts of automaticity and two of its dimensions- unintentionality and efficiency. Impulsivity was unrelated to the level or change of PA automaticity. Discussion: A process of PA habit formation, conceived in terms of automatic properties, is likely to be easier when the PA is performed at least 3 times a week (high frequency) and when individuals choose to adopt active transportation modes (low complexity). Future research should examine the role specifically played by personality traits in the increase of PA automaticity.