In this paper, we examine how private regulatory initiatives (PRIs) – which define standards for corporate responsibility (CR) issues and sometimes monitor their application by firms – create opportunities and constraints for activist groups aiming to push firms toward more stringent CR activities. Drawing on social movement theory, we conceptualize how private regulation opportunity structures affect such CR‐based activist groups’ targets and tactics at both the firm and field levels. At the field level, we argue that both radical and reformative activist groups direct most of their time and resources towards PRIs with comparatively more stringent standards. At the firm level, while radical activist groups are likely to target firms participating in more stringent PRIs, reformative activist groups target firms participating in less stringent PRIs, or those that do not participate in PRIs at all. When facing unfavorable opportunity structures, CR‐based activist groups tend either to advocate the creation of new PRIs or to shift their activities to pressure other focal points. This paper contributes to moving beyond extant literature's emphasis of PRIs as settlements of contentious firm‐activist interactions towards also viewing them as starting points for activist groups aiming to push firms toward more substantive CR engagement.