The article offers an edition and commentary of a previously unknown constitution of Gratian, discovered on the final folio of Arras, Médiathèque Municipale MS 644 (CGM 572). Although severely abridged and difficult to read, this newly identified document addresses the status of formerly abandoned children (expositi) who have been taken in within an ecclesiastical context. It establishes that the biological father – or, in the case of slaves, the original master – can no longer reclaim them. This law is best understood within the broader context of late antique legislation on expositi, while early Christian sources illuminate the historical circumstances that may have prompted such legislation. Finally, the hypothesis that this law may have originated from the Theodosian Code is examined. An appendix highlights the utility of prose rhythm in establishing the text.