This article explores the production and consumption of phrenological knowledge for and by middle-class women in the USA during the early and middle decades of the 19th century. At a time when science itself had few boundaries, women became readers, consumers, proselytizers and practitioners of this knowledge system, outside of a scientific academy. This paper argues that phrenological beliefs about sex differences enabled and encouraged women to be users. Phrenology allowed women to negotiate gender and by encouraging followers to ‘know (...) thyself,’ phrenology blurred the lines of expertise, creating a fluid interplay between users and producers of knowledge. This article then shows how categories of women users–practitioners, consumers and feminists–implemented or rejected elements of phrenology as they sought to affirm or amend prescribed gender roles. As the industrial economy seemed to divide public and private, production and consumption, masculine and feminine, phrenology allowed women and men to stand within and between these binaries. At the same time, some women used phrenology to classify themselves and others in a socio-natural hierarchy and further engrained scientific racism in American culture. (shrink)