This study explores ways to support girls of color in forming their senses of selves in science, technology, engineering, and math (STEM) during the middle school years. Guided by social practice theory, we analyzed a large data set of survey responses (n = 1,821) collected at five middle schools in low-income communities across four states in the United States. Analyses focus on the extent to which key constructs that inform girls' development of senses of self and relations among those indicators of STEM identities varied by their race/ethnicity. Though the means of indicators sometimes varied across racial/ethnic groups, multigroup structural equation modeling analyses indicate no significant racial/ethnic differences in the relations of STEM identities, suggesting that similar supports would be equally effective for all girls during the middle school years. Girls' self-perception in relation to science was the strongest predictor of their identification with STEM-related careers, and this self-perception was positively and distinctively associated with their experiences with science at home, outside of school, and in school science classes. This study argues for strategically expanding girls' experiences with science across "multiple" settings during middle school in a way that increases their positive self-perception in and with STEM.