The value of MS Paris, BNF, lat. 18275 is manyfold. This paper gives a new, exhaustive description and a precise dating (c. 1170), and suggests that the MS may have been copied by or for the jurist Placentinus, maybe in Southern France, or in Northern Italy – and anyway, it used French as well as Italian sources. This MS is a key witness to the tradition of Ausonius, and, in particular, it alone transmits two extracts, so far neglected, that belong most probably to Ausonius.