The study explored the relationship of self-concept and locus of control to science engagement and science achievement. Furthermore, the relationship of self variables with science engagement and science achievement was studied across ethnicity, gender, and socioeconomic status (SES). The data were accessed from the 3 waves of the National Education Longitudinal Study: 88 (NELS:88) to estimate both cross-sectional and multilevel longitudinal models. It was hypothesized that self-concept and locus of control would have a positive effect on science engagement and science achievement and that science engagement would affect science achievement positively. The results supported the theoretical formulations of the conceptual model and showed significant effects of self-concept and locus of control constructs on science engagement and science Item Response Theory (IRT) scores. Furthermore, science engagement showed a positive but small effect on science achievement, especially the time spent on science homework. The study supported earlier findings that the inclusion of non-ability factors improves the explanation and understanding of differences in science achievement. The study has implications for practice. (Contains 7 tables and 4 figures.)