Brazilian prisons are typically crowded and poorly resourced, yet at the same time may be active places. Of particular interest to the sociology of prisons is institutional reliance on inmate collaboration and self-ordering, not only to maintain prison routines, but, in the most low-staffed prisons, security and prisoner conduct as well. This article explores the roles played by inmates in running one such penal institution, a men's police lockup in Rio de Janeiro. At the time of research the lockup had over 450 prisoners, but just five officers. Both on and off the wings inmates performed janitorial, clerical, and guard-like duties, mostly under the supervision not of officers but other prisoners. The lockup appeared to be operating under a relatively stable, if de facto and provisional order, premised on common needs and shared beliefs, and maintained by a hierarchy of prisoner as well as officer authority.