Several Qur’anic and Prophetic statements appear critical of Christian monasticism, most famously, perhaps, the Hadith “there is no monasticism in Islam.” In spite of this critical attitude, however, many early Muslims held Christian monks in high esteem and recognized affinities between Islamic and Christian ascetic practice. This talk argues that Muslim ascetics and Christian monks learned from and adapted each other’s ascetic ideals in both critical and appreciative ways. The talk begins by introducing the place of Christian monasticism in the early Islamic world, as well as the development of Islamic attitudes toward monasticism. I then focus on a series of related traditions, which appear in both medieval Islamic and Christian sources, concerning the black robe worn by Christian monks. This visible symbol of piety was used by Muslim and Christian authors to convey shared ideals of asceticism, as well as to critique the practices of rival groups. By examining the interreligious transmission of stories about the black robe, the talk reveals the mutual exchange of ascetic ideals across Muslim-Christian lines. This exchange, I argue, played an important role in the formation of both Islamic and Christian religious practices in the medieval Middle East.