This article reconstructs the social ecology of the Brazilian capital city's principal Detention Center (Casa de Detenção) at the beginning of the country's First Republic (1889–1930). Most of the persons in Rio's city jail at this time were only detained for a relatively brief period and without any formal charges. This detention center's manuscript entry logs, annual ministerial reports, administrative correspondence, and published prison diaries and journalistic accounts reveal this institution as a living theater that dramatizes the gap between legal code and real-life juridical practice more powerfully than perhaps any other institution in modern, urban Brazil. The criminal justice system provided the primary interface between the state and nonelite members of Rio society. During their time in the Casa de Detenção before being acquitted, transferred to a different penal facility, or deported, an astonishingly heterogeneous group of detainees interacted with each other and with agents of the state. Ultimately, I argue, detainees' experiences in the city jail provided them with a civic education of sorts; in this setting, inmates—and indirectly their families and associates outside the jail—learned not only how to navigate the criminal justice system but also, more generally, the informal and formal rules that governed their society. Going beyond the assumption that the incarcerated were socially "dead," this article seeks to contribute an understanding of the social ripple effects of informal judicial and policing procedures in urbanizing, post-abolition Brazil in the nineteenth and twentieth centuries.