In drawing a link between Marseille and Algiers, one a colonial city and the other metropolitan, this paper seeks to demonstrate the importance of Mediterranean networks of knowledge for understanding colonial police practice. Police officers in both cities wove a spatial understanding of black market activity that relied on a racialized vision of urban space. In Marseille, the police efforts to repress the black market focused on Rue des Chapeliers, the heart of Marseille’s North African immigrant neighborhood. In Algiers, the police scoured the streets of the Casbah. In seeking “anti-French” Algerian black marketers, the police arrested individuals, but they approached Algerians as a collective threat, targeting neighborhoods the police understood as Algerian. Reading “against the grain” of police reports, this paper explores how the police themselves helped create the imaginary of the Algerian black marketeer and how the tactics police used to control this supposed threat brought them into the lives of ordinary Algerians whose connection to a shadowy black market world was tenuous at best.