Environmental disasters or other large-scale disruptive events often trigger the emergence of social movements demanding social and/or political change. This study investigates mobilization processes at the meso level of the Japanese anti-nuclear movement after the nuclear disaster at the Fukushima Daiichi nuclear power plant caused by the Great East Japan Earthquake and subsequent tsunami waves on March 11, 2011. To capture such meso level movement dynamics – which so far have played only a minor role in research on social movement mobilization – the study presents an analytical model based on premises from political process theory, network theory, and relational sociology. This model is then applied to the case of the Japanese anti-nuclear movement after Fukushima by looking at the relational dynamics of two coalitional movement networks engaged in advocacy-related activities in Tōkyō.