Both ancient writers and modern scholars have noted the frequency with which ascetics in the late Roman Empire were accused of Manichaeism. Such accusations are often discussed in relation to specific incidents, but have not typically been examined as a phenomenon in their own right. In this thesis, I compare the roles of Manichaean accusations in the Priscillianist and Jovinianist controversies. Priscillian was convicted of sorcery, and yet the accusation of Manichaeism haunted him so thoroughly that after his death, the emperor Magnus Maximus freely referred to him and his companions as Manichaeans. Prior to his death, Priscillian condemned Manichaeism for its illegality and idolatry, suggesting that Manichaeans were sun-worshippers in documents intended to demonstrate his own orthodoxy to skeptics. In the Jovinianist controversy, a veritable web of accusations plagued participants on both sides. The controversy’s particular focus on marriage, sex, and celibacy created an environment in which Manichaeism was frequently invoked, first to condemn anyone who rejected marriage as the Manichaeans were said to do, and later to condemn even Jovinian, who specifically campaigned against the elevation of celibacy. A comparison of the two controversies reveals a few basic patterns. First, the “Manichaean” label was versatile because Manichaeism itself carried a wealth of negative connotations, from magic to rejection of marriage to idolatry; it could easily be applied to opponents for a broad variety of reasons. Second, there are commonalities between the accusations in the two controversies: association with illegality, heresy, and asceticism are the common threads that unite the accusations. Third, in a time when the boundaries of orthodoxy were still being debated, Manichaeism served as a useful signpost for what was not orthodox. There were many polemical labels that could be applied to one’s opponents in this period; I argue that the accusation of Manichaeism stood out for its versatility, power, and the immediate relevance it gained from the visible presence of Manichaeans in the Empire.