Although the gender gap has dramatically narrowed in recent decades, women remain underrepresented in many science, technology, engineering, and mathematics (STEM) fields. This study examined social and personal factors in relation to adolescent girls' motivation in STEM (math/science) versus non-STEM (English) subjects. An ethnically diverse sample of 579 girls ages 13-18 years (M = 15) in the U.S. completed questionnaires measuring their academic achievement, ability beliefs, values, and experiences. Social and personal factors were hypothesized to predict motivation (expectancy-value) differently in math/science (M/S) and English. Social factors included perceived M/S and English support from parents and peers. Personal factors included facets of gender identity (felt conformity pressure, gender typicality, gender-role contentedness), gender-related attitudes, and exposure to feminism. In addition, grades, age, parents' education, and ethnicity were controlled. Girls' M/S motivation was positively associated with mother M/S support, peer M/S support, gender-egalitarian beliefs, and exposure to feminism; it was negatively related to peer English support. Girls' English motivation was positively associated with peer English support as well as felt pressure from parents; it was negatively related to peer M/S support and felt peer pressure. The findings suggest that social and personal factors may influence girls' motivation in domain-specific ways.