Alone among the French romances of Alexander the Great penned in the twelfth century, Thomas of Kent’s Roman de toute chevalerie reproduces the story of Alexander’s illegitimate birth from the principal Latin source. According to this account, Alexander’s father was Nectanabus, a mage and astrologer who seduced queen Olympias with an astronomy lesson, deceived her by using animal pelts to disguise himself as a god, and then used his magic arts to retard the child’s birth when his astrological calculations indicated the child would be born a hybrid man-beast. Thomas wrote his romance at the very moment when both astrology and paradoxography (the writing of marvels) were being reevaluated as means of understanding the world, and so Alexander’s odd birth offers a reflection—shaped by the romance genre—on the limitations and ethical implications of medieval natural science.