In this paper, we examine the behaviour of so-called passive and middle aorist forms in the Greek reflected in the Genesis of the Septuagint. The Septuagint, and Biblical Greek more generally, displays a considerable aberration with respect to other varieties of Ancient Greek regarding the relative frequency of passive vis-a-vis middle aorist forms. Here, we explore this feature of Septuagint Greek in some detail, showing that there is a more or less consistent, binary system at this stage, where the passive aorist is the preferred expression of monoargumental detransitive clauses, while the middle primarily appears in biargumental detransitive clauses. A comparison with the original text in Biblical Hebrew suggests that its binyanim system, with systematic distinction between transitive and detransitivising stems, may have played a key role in accelerating this process, which was already present in earlier stages of Greek. This process resulted in a general ousting of the aorist middle, which essentially became a lexically restricted variant of the active voice.