Euthydemus is included neither in Die Fragmente der Vorsokratiker by Diels–Kranz nor in Sofisti. Testimonianze e frammenti by Untersteiner nor in Early Greek Philosophy by Laks and Most. Likewise, the great twentieth century works on the Sophists do not give space to him, at best mentioning him briefly. Yet Euthydemus is the author of a fragment, which was quoted by Plato in his Cratylus, and on which again there is no modern study. This paper sets out to study this (...) fragment in depth, to review and discuss the various translations, both existing and possible, to study the context of Plato’s quotation, to clarify the exact meaning and the theoretical scope of this important fragment, and lastly to place Euthydemus in the Protagorean tradition, of which he is a notable product. (shrink)