Science, technology, engineering, and mathematics (STEM) are growing fields in both global job markets and educational spaces. The problem related to this study was the lack of understanding of how gender and ethnicity might relate to differences in the science self-efficacy, outcome expectations, and task interest of students who have participated in STEM intervention programs at the middle school level. The purpose of this quantitative study was to explore the extent to which there were differences between the dependent variables of science self-efficacy, outcome expectations, and task interest in U.S. middle school students based on the independent variables of gender and ethnicity after participating in a citizen science STEM intervention program. Social cognitive career theory was the theoretical framework for the study. This study was a nonexperimental comparative investigation based on survey responses from students who had participated in a water quality, citizen science STEM intervention from 2017-2019. The participating students’ school district has a history of multiple, systemic STEM learning experiences. The results of two-way MANOVA indicated that there were no statistically significant differences in career choice variables between male and female students and between non-Hispanic and Hispanic students after participating in a citizen science intervention program. This study has the potential to help students from underrepresented populations to envision success in their STEM educational and career pathways by seeing other students experience success in those areas. Educators may also be better able to design programs that address the specific needs of underrepresented student populations, which may lead to better student outcomes for those groups.