At a later date he took part in the theological discussions of the age and is the author of "Inventaires des faultes, contradictions, faulses allégations du Sieur Plessis, remarquées en son livre de la Sante Eucharistie, par les théologiens de Bordeaux" (Bordeaux, 1599–1601).
Librarian from 1604 of the Collège de Clermont, he reorganized the library, which had been scattered during the period in which the Jesuits had been obliged to abandon the school.
At the suggestion of Casaubon, Henry IV of France contemplated the publication of manuscripts of the royal library.
The clergy of France decided to confide the revision of the Greek Fathers to the Jesuits, and Fronton du Duc was chosen by the Society to labour on this project.
This edition follows a Vienna manuscript that had belonged to the library of Matthias Corvinus; its publication had been delayed by a series of complications in which the political schemes of Richelieu were involved.