Based on direct observation of police work in a Parisian suburb, this paper aims to analyse the circumstances in which police discrimination takes place. The ethnographical approach allows us to highlight the process of categorization and the routine police practices that, in certain cases, modify the framework of police encounters and transform the interaction with officers of the law into a discriminatory relationship. The analysis in terms of 'discriminatory power' suggests moving beyond the normative approach to the study of discrimination, and restoring the dynamic and relational element to interactions between police officers and members of the public.