The concept of biological sex guides research, clinical practice, science funding policy, and contemporary political discourse. Despite some substantive differences, all existing candidate philosophical accounts of sex assume its legitimacy as a biological concept. Here, we challenge this view. We argue against realism about biological sex, and that eliminating biological sex from large swaths of biological theory and practice may be preferable compared to conventionalist or fictionalist anti-realisms. There are serious social and epistemic costs to using “biological sex” in place (...) of more specific alternatives. Because of this, biologists and philosophers of science should consider eliminativism about the concept of biological sex. Biological sex eliminativism is worth taking seriously, and it can play important roles in philosophical debate and biological practice, even for those who remain skeptical. The methodological consequences of biological sex eliminativism are compatible with best practices for inquiry in the biological and biomedical sciences, with inclusive approaches to the study of sex and gender, and with feminist philosophical and methodological recommendations. Taking eliminativism seriously reveals important disagreement about the work that a concept of biological sex should do, and imposes a contrastive burden on would-be rivals. (shrink)