This study addresses the role of domain-general mechanisms in second-language learning and knowledge using an individual differences approach. We examine the predictive validity of implicit-statistical learning aptitude for implicit second-language knowledge. Participants (n = 131) completed a battery of four aptitude measures and nine grammar tests. Structural equation modeling revealed that only the alternating serial reaction time task (a measure of implicit-statistical learning aptitude) significantly predicted learners' performance on timed, accuracy-based language tests, but not their performance on reaction-time measures. These results inform ongoing debates about the nature of implicit knowledge in SLA: they lend support to the validity of timed, accuracy-based language tests as measures of implicit knowledge. Auditory and visual statistical learning were correlated with medium strength, while the remaining implicit-statistical learning aptitude measures were not correlated, highlighting the multicomponential nature of implicit-statistical learning aptitude and the corresponding need for a multitest approach to assess its different facets.