This paper analyzes levels of environmental activism observed during a two year period in the early 1990s in the municipalities of the six Mexican states located in the United States-Mexico border. By focusing on a short period of time, it aims to capture the relevance of opportunities for mobilization. It uses multivariate statistical methods to relate variations in conflict rates to economic and political conditions in the area. It shows that even when using aggregate data it is possible to identify a pattern of concentration of activism following the geographical distribution of economic growth. The paper identifies that in this region the two important factors shaping the patterns of environmental activism are the influence of border dynamics and the presence of environmental groups with the experience and capacity to form international coalitions. The analysis demonstrates that these patterns of activism may eventually influence the geography of environmental pollution in the region.