By the time that Hannah Arendt's The Origins of Totalitarianism was published in 1951, one totalitarian regime lay in ruins while another–Soviet Communism–stood newly reenergized. Stalin's prestige, burnished by victory, had never been greater. The cold war was re-dividing the world. And, in the United States, the fear of Communism was a pervasive feature of the political landscape. The revelation that the State Department and other government ministries, in the 1930s and beyond, had been penetrated by American Communists, prompted public outrage against the miscreants. It aroused corresponding curiosity about those who had once embraced the Marxist creed but who now publicly renounced it. One such person was Whittaker Chambers (1901–1961)–Communist spy, Communist defector, and key witness in the perjury trial of Alger Hiss. Into this imbroglio stepped Hannah Arendt. She was by turns suspicious and dismissive. Who were the ex-Communists? Why had they broken with Communism? How thorough or deep-rooted was their abrogation? On her account, Chambers and others like him had no understanding of democratic politics and of what it means to be a quotidian political actor. She also denounced ‘ex-Communist’ informing as analogous to the exposure practices of totalitarian regimes. This article evaluates the cogency of her analysis. It devotes particular attention to a problematic distinction Arendt draws between ‘ex-Communists’ and ‘former’ Communists. And it seeks to answer a question that Arendt left opaque: when, if ever, is informing against fellow citizens justified in a constitutional republic?.