French materialist feminists such as Christine Delphy and Monique Wittig maintain that the social fact of women’s exploitation by men within the family pre-exists and produces gender differences as well as the perception that men and women belong to different biological sexes. They take this position to be ‘materialist’ because it puts social facts prior to ideas and beliefs and so puts the ‘material’ prior to the ‘ideal’. However, I shall claim, drawing on arguments of Sebastiano Timpanaro’s, that this is an incomplete form of materialism because it neglects the shaping of social facts by their interaction with biological facts, notably the biological difference between the sexes. Wittig, though, denies that there is any ‘fact’ of biological sexual difference. Wittig claims that we only believe in and perceive two biological sexes due to the influence of gender expectations. I will try to show that Wittig’s arguments for this claim undermine themselves and actually presuppose that there are two biological sexes. I will conclude that, given that there are two biological sexes, a fully materialist form of feminism must take account of sex difference in theorising gender inequalities.