In this article, we challenge the notion that aptitude--operationalized as grammatical sensitivity as measured by the Words in Sentences section of the Modern Language Aptitude Test--is central to adult second language (L2) acquisition. We present the findings of a study on the acquisition of two properties of Japanese, head-final word order and case marking, by naïve learners of Japanese as a L2 who had no prior knowledge of this language or any other head-final, case-marking language. Participants underwent an input treatment in which they heard and saw basic subject-object-verb sentences in Japanese and were subsequently tested on these basic sentences and also on sentences to which they were not exposed during the treatment (i.e., polar questions and embedded clauses). Reading times on grammatical and ungrammatical sentences served as measures of underlying sensitivity to violations of grammatical word order for both the basic sentences and the novel sentences. Our results yielded three groups of learners: those who showed sensitivity to violations of basic word order, polar questions, and embedded clauses (parameter reset); those who showed sensitivity to violations of basic word order and polar questions only (partial parameter reset); and those who showed sensitivity to violations of basic word order only (no parameter reset). Because our measure of aptitude as grammatical sensitivity did not emerge as a factor distinguishing the three groups from one another, we argue that this component of aptitude is not a factor in whether learners are able to reset parameters. We also found that all participants, regardless of group, demonstrated sensitivity to case-marking violations, suggesting that aptitude as grammatical sensitivity plays no role in the acquisition of underlying features related to case or to the surface-level markings of case in Japanese.