The article deals with the unique phenomenon of the socio-political life in the USSR in the 1930s — functioning of political opposition to the existing regime under prison conditions. Expelled in the late 1920s from the Communist party and labeled “anti-Soviet”, various oppositional groups, among which Trotskyists (“Bolsheviks-Leninists”) constituted a majority, tried to operate clandestinely, but soon most of their activists have been arrested and exiled or imprisoned. Core cadres of imprisoned oppositionists were concentrated in the Verkhne-Uralsk political isolator, were they formed the “Communist sector”. On the basis of various sources, including manuscripts found in 2018 in Verkhneuralsk prison’s hiding- place, the article discusses main aspects of life and activities of Communist oppositionists in prison: forms of their self-organization; struggle for “political regime” in custody; characteristics and evolution of their ideological views; elaboration of programmatic and tactical principles; factional discussions, splits and fusions; publication of prison periodicals. In the Soviet Union the Verkhne-Uralsk’ and other political isolators constituted a kind of oases of dissidence and political pluralism, that predetermined their liquidation by the mid-1930s under conditions of the Stalinist regime’s tightening on the eve of the Great Terror.