PIETRO, DEACON. — Librarian and archivist of the abbey of Montecassino, born c. 1107, died in 1159. Under the guidance of the monk Guido, P. acquired a solid erudition in sacred sciences and history. Perhaps because of his restless spirit he was away from Montecassino for some time; but having returned to his abbey, at the behest of Abbot Seniore and, even more, driven by an inner literary passion, he set to work with surprising alacrity on various hagiographical and historical writings, not without sometimes altering oral and documented tradition to suit his own purposes.
Among these works is remembered the continuation of the *Chronicon Casinense* of Leo of Ostia, in which he also used the work of his master Guido and at the same time made changes that were not always felicitous to Leo’s work as well. Other published works of his are: *Liber illustrium virorum archisterii Casinensis*; *De ortu et obitu iustorum Casinensium*; *Liber de locis sanctis*; PL 173. Also found there is the part he wrote of the aforementioned *Chronicon Casinense*, reproduced from the edition by Wattenbach in MGH, *Script.*, VII, p. 551. Other writings of his remain unpublished in the Cassinese codices; among these, worthy of mention is the *Registrum*, in which P. collected abundant archival material to vindicate the patrimonial rights of his monastery. He himself recounts the defense of these rights in the presence of Emperor Lothar, who decided the controversy in favor of Montecassino.