Aptitude-treatment interaction (ATI) research is of both theoretical and practical interest to second language (L2) learning, since it provides insights into the processes linking learner-internal individual difference factors and learner-external contextual variables including instructional approach--variables that jointly determine L2 outcomes. The present study employed a full range of aptitude measures mapped onto four explicit instructional conditions: auditory inductive, written inductive, mixed inductive, and mixed deductive. International volunteers (N = 136) completed online language lessons in beginners' Polish targeting two morphological features. Participants' phonetic and language-analytic abilities, level of multilingualism, and age predicted L2 achievement. A cluster analysis identified four learner profiles: high aptitude, low aptitude, memory oriented, and analytically oriented. Deductive instruction seemed to neutralise individual differences in aptitude, while ATI effects were observed in the single-modality conditions, with auditory input favouring high-aptitude learners and written input favouring high-aptitude, analytically oriented, and memory-oriented learners. We discuss the theoretical and practical import of these findings by highlighting the "capital" afforded by prior language learning experience, over and above the role of cognitive ability. In addition to the inductive-deductive contrast in explicit instruction, we emphasise the importance of input modality, which has hitherto been neglected in the field.