My premise is that a transmasculine experience of embodiment haunts the subject of The Second Sex. In light of her early philosophical essay on Claude Bernard, Beauvoir's account of sex difference may be read as the biological experience of “error” that foils what biopolitics and early twentieth‐century knowledge in the life sciences wanted to make of her. I argue that from this perspective the crucial discussion of the endocrine system of the human female in the Biology chapter adds a new (...) twist to the medical and scientific discourses for understanding transsexual identification. (shrink)