Regardless of this decision and infringing on the rights of his colleague of Arles, Mamertus consecrated in 463 a bishop for the city of Die (Dea).

In a subsequent letter to the bishops of the Roman provinces of Lyons, Vienne, Narbonnensis I and II and Alpina, he also refers to the matter, and directs them to obey Leontius's summons to a regularly constituted synod.

Hilarius therefore commissioned Veranus of Vence to inform Mamertus that, if he did not recognize and submit to the regulations of Pope Leo, he would be deprived also of the four suffragan dioceses, still subject to Vienne.

[1] During his episcopate, the remains of Ferreolus were discovered, and were translated by Mamertus to a church in Vienne, built in honour of that martyr.

in P. L., LIX, 289-94) Bishop Mamertus was the founder of the Minor Rogations Processions, held on Monday to Wednesday preceding Ascension Thursday.

[6] The Minor Rogations were traditionally observed with processional litanies and fasting as a petition for good weather for the crops and deliverance from pestilence and famine.