This cross-lagged longitudinal study was conducted with 862 seventh and eighth graders (secondary school) in the province of Québec (Canada) to study the effects of two important perceptual variables (self-concept and individual interest) on achievement, as well as reciprocal relations between all these constructs. Considering the results obtained previously in mathematics education, it was designed to test if the same inter-variable dynamics could be recorded in science and technology. The data was gathered at 10 time points (four perceptual; six report cards [school reports]) and analyzed using Mplus. Most fit indexes were acceptable and revealed a predictive solution that supports the hypothesis that interest does not appear to play any direct role in achievement, but that self-concept does. Recommendations for research that tests individual interest are formulated as well as suggestions for educational practice.