The first essay of Nietzsche’s On the Genealogy of Morals seeks to uncover the roots of Judeo-Christian morality, and to expose it as born from a resentful and feeble peasant class intent on taking revenge upon their aristocratic oppressors. There is a broad consensus in the secondary literature that the ‘slave revolt’ which gives birth to this morality occurs in the 1st century AD, and is propogated by the inhabitants of Roman occupied Judea. Nietzsche himself strongly suggests such a view. (...) However, in a telling later passage from Ecce Homo, Nietzsche claims that the hsitorical Zarathustra—a Bronze age iranian religious thinker—was the first to consider the opposite of Good vs. Evil; that “Zarathustra created this most fateful of errors, morality” (EH, 'Destiny', §3). However, Nietzsche does not discuss Zarathustra or Zoroastrianism in his critique of moral values and their origin. This creates a prima facie tension. If at least part of what essentially characterises 'morality' preceded Judeo-Christianity, are moral values only contingently related to the feelings of ressentiment essential to Nietzsche’s story of the slave revolt? If the answer is 'yes', then the scope of Nietzsche's critique of morality may be somewhat limited. If the answer is 'no', then we require an answer as to why Nietzsche's genealogy—if it is an exercise in ascertaining the “real history of morality” (GM, Pref: §7)—does not extend further back in history. In this paper, I explore how the extent of Nietzsche's knowledge of Zoroastrianism informs his critique of slave morality in On the Genealogy of Morals. I argue that Nietzsche views the historical Zarathustra—like Socrates and Plato—as a forerunner of ‘morality’ in his creative conception of good and evil in metaphysical terms. From here, it is argued that the proposed tension can be dissolved by viewing Judeo-Christian morality merely as the latest and paradigmatic expression of slave morality. (shrink)