Women contend with gender bias in certain science contexts, which suggests they may be likely to adopt prevention-focused modes of regulation aimed at maintaining safety and security in such settings. This study represented an integrated test of regulatory focus theory (RFT; Higgins, 1997, 1998) and the critical mass hypothesis, which assumes that women are attracted to scientific career fields in which their group represents a majority. Participants were 255 female undergraduate science, technology, engineering, and/or mathematics (STEM) majors enrolled in science laboratory classes. Results indicated that number of men in labs moderated the mediated relationship between affiliation and intrinsic science motivation via academic satisfaction. This moderated mediation effect was observed at low, but not average or high, levels of men in labs. No such effect was observed at high levels of women in labs. Theoretical and empirical implications for research related to the underrepresentation of women in STEM are discussed.