Finding the match between individuals and educational treatments is the aim of both educators and the aptitude-treatment interaction research paradigm. Using the latent growth curve analysis, the present study investigates the interaction between the type of explicit instructional approaches (deductive vs. explicit-inductive) and the level of foreign language aptitude (high vs. low) in the learning of explicit grammar rules. The results indicate that on the whole the two equally explicit instructional approaches did not differentially affect learning performance. However, when the level of language aptitude, measured by grammatical sensitivity, associative memory, and memory for text (with the last variable being the best measure), was taken into account, low-aptitude learners performed significantly better with the deductive instruction, in the sentence-correction tests. The interaction effects of equally explicit instructional approaches suggest the need for considering aptitude-treatment interaction to maximize learners’ potential for success in second language learning.