This paper explores various relationships between categorization processes and urban public spaces. Categorization processes are a central phenomena contributing to the organization and the intelligibility of social conducts. These processes were described by Harvey Sacks in an ethnomethodological and conversational perspective; in this paper they are tackled within the same framework, articulated with some issues raised both by the urban studies and the urban sociolinguistics. Cities are places where categorization practices occour, exploiting a variety of action formats and of verbal and non-verbal resources. Their analysis is first based on the figures of the Stranger in Simmel and the Chicago School texts; then it goes on by evoking some ethnomethodological analyses of glances and talk between unacquainted people in the public spaces; finally it develops some analyses of the social categories used in urban narratives.