•Theory suggests that transparency interventions are less likely to succeed when corrupt actors are politically influential and the victims of corrupt acts confront large obstacles to collective action.•We investigate a transparency treatment in just such a setting, looking at the behavior of large and politically connected contractors who supply school meals in Colombia.•A treatment that combines two interventions, informal audits of the meals provided and a series of text messages to parents informing them about their options for registering concerns, significantly reduces contractor shirking. Can transparency interventions reduce corrupt behavior when corrupt actors are politically influential and the victims of corrupt acts confront large obstacles to collective action? These conditions describe the pervasive phenomenon of grand corruption and potentially render corrupt actors less vulnerable to transparency interventions. We present the first evidence that, despite these theoretical obstacles, a transparency intervention in the Colombian School Meals Program significantly changed the behavior of powerful operators. The intervention consisted of informal audits and text messages to parents. It affected behavior through two channels. A survey of parents reveals greater bottom-up mobilization to oversee operators in treated schools; the pattern of operator responses to the informal audits over time and across departments indicates that operators were concerned that systematic evidence of corrupt behavior would trigger top-down enforcement actions by high-level enforcement agencies.