This paper analizes the tragic genealogy of Fortune and Nemesis in book 14 of Ammianus Marcellinus. Specifically, the motif of the fickle Fortune that appears in the episode of Caesar Gallus is compared with its presence in Greek and Roman tragedy. In turn, the ambiguous nature of Fortune created by the introduction of Nemesis at the end of the book will be shown and the ways in which this duality is created will be analized. The comparison of similarities and differences of Ammianus' work with tragic authors may allow a better understanding of the author's style. The general aim of this work is to delve into the literary texture of Ammianus' Res gestae, under the assumption that understanding his style may help to refine possible conclusions about his historiographical method.