This thesis deals with the arithmetical commentary by Abbo of Fleury on the series of multiplication tables ascribed to Victorius of Aquitaine (fl. 450): the Explanatio in Calculo Victorii. My research aims to shed light on the history of medieval Neopythagoreanism between the 10th and 11th centuries, in which Abbo’s work stands as a peculiar testimony of what my work indicates as “theological arithmetism”. The organization of my thesis follows this overall structure. In the first two chapters Abbo’s life and works are presented with a special emphasis on his educational training and learning. Then, the main topics of his “mathematism” are presented, starting with Abbo’s interpretation of the Biblical passage on God who created everything in number, wheight and measure (Wisd. 11, 21), which is discussed at chapter 3, also by illustrating its philosophical background. Then, I present Abbo’s “henology”, namely his consideration of Unity, with regard to both divine and created unity (chapter 4); moving next to his conception of ontological and material compositeness, which Abbo ascribed to creatures (chapter 5) and finally to his arithmetical and computistical knowledge grounded on Boethius’ number theory and his methods of reckoning (chapter 6). To this overall exposition of Abbo’s Neopythagoreanism, it follows a section devoted to Peripatetic approach to the analysis of the natural phenomena (chapter 7). The methodology applied to my research intends to place Abbo’s thought in its cultural context: the renewal of the liberal studies and particularly of the quadrivium, which was stimulated by a progressive rediscovery of Boethius’ De arithmetica together with the introduction of new calculation tools in the Latin West. If this is the general background from which Abbo’s peculiar “Pythagorism” emerges, the philological analysis of his commentary confirms how Fleury was an outstanding centre for high-level mathematical studies. The manuscript tradition of both Abbo’s Explanatio and Victorius’ Calculus, let us envisage two possible paths through which the latter reached Fleury Abbey. A final goal of my work is also to consider Abbo’s commentary within the history of theoretical arithmetics during the first Millennium. Tracking Abbo’s philosophical sources and showing his distinctive comprehension and harmonization of different Platonic accounts, namely those spelt out in Mamertus’ De statu animae, Calcidius’ Commentary on Timaeus, Macrobius’ Commentary on the Dream of Scipio and Saturnalia and Boethius’ De arithmetica and De institutione musica, has been a stimulating research and a confirmation of Abbo’s outstanding role in the not yet sufficiently explored history of high medieval “mathematical” approach to theology and world vision.