Philo's Quaestiones are exegetical pieces on individual points of interest in the text of the books of Genesis and Exodus, using various traditions and exegetical approaches(Jewish and Greek). Following the direction of the text Philo rarely lets an opportunity pass to interpret the meaning of a number mentioned in the biblical text. Philo mentions a book of his on the properties of numbers (QGen IV,110), and there have been attempts at reconstructing this lost treatise (Staehle). This paper addresses the issue of whether this book is the only source of his arithmological exegesis in the Quaestiones. It studies the use he makes of arithmology in these particular writings and his use of sources for his arithmological expositions. These are then compared with Philo's use of arithmology in other treatises on the same biblical texts. The aim is to shed light on the following quesions: Did Philo merely use his own treatise on the numbers, did he apply general arithmological material or were there specific, already existing collections of arithmological interpretations of the numbers mentioned in the bible, comparable to onomastica of biblical names?