G. E. R. Lloyd has argued that Parmenides ‘probably held that the sex of the child is determined by its place on the right or left of the mother's womb (right for males, left for females)’. It is the purpose of this paper to challenge this assertion by re-examining the primary evidence of fragments 17 and 18 of Parmenides as well as the tangled mass of testimony of the doxographers, Censorinus, Aëtius and Lactantius. In so doing I shall consciously observe a sharp distinction between theories of sex differentiation and theories of heredity since I shall argue that the confusion of the two subjects has led to distortion of Parmenides' doctrines.