During the “trial” of Caelestius in Carthage, 411, written evidence was produced by his accusers. We are not, unfortunately, informed of the content of this evidence. Again, in the trial of Pelagius in Diospolis, 415, evidence presented against him was taken from a Liber Caelestii. Of this we are given some summary account, but little attention was given to it, mainly because Pelagius rejected its relevance in case against himself. Although some “extracts” of this Liber were included in the Acts of Diospolis, the citations are not literal. In his examination of the Acts (De Gestis Pelagii), Augustine notes that he had a “similar” book, from which he gives us some extracts. Earlier (413?) when examining a collection of syllogisms attributed to Caelestius (De Perfectione Iustitiae Hominis) Augustine could say that he recognised the style and thought of Caelestius in the collection, because he had read a writing by Caelestius, but this writing seems to have been circulated anonymously. In his first anti-Pelagian work (De peccatorum meritis et remissione) Augustine refuted a series of Pelagian theses that indirectly, through their use by Pelagius, we can attribute to Caelestius. Modern collections of the fragments of Caelestius’ writings do not include these. In this paper I will argue that the Liber Caelestii is not in fact lost, but that some sizeable fragments remain, and that the general outline of this foundational text for Pelagianism can be reconstructed.