The purpose of this study was to determine if attending a single-sex, STEM-focused school was related to science and mathematics achievement among a sample of African American and Latina eighth-grade girls in one school district. A nonequivalent group, post-test only design was utilized in the study to mitigate the effects of selection bias. We analyzed secondary data using multiple regression, controlling for race/ethnicity, socioeconomic status, and prior mathematics achievement. The results suggest that students who attended the single-sex middle school scored significantly higher than students who attended the co-educational middle schools in mathematics and science.