In recent work by Tsimpli (2003) and Tsimpli and Dimitrakopoulou (to appear) an explicit claim is made about the nature of end-state grammars in older second language (L2) learners: uninterpretable syntactic features that have not been selected during first language (L1) acquisition will not be available for L2 grammar construction. Interpretable syntactic features, on the other hand, remain available (as well as the computational procedures and principles of the language faculty), even those not selected by the L1. The present study investigates this hypothesis in relation to the acquisition of the uninterpretable feature that forces "wh"-movement in interrogatives in English. Nineteen L1 speakers of Japanese (a "wh"-in-situ language that lacks the movement-forcing feature) who are highly proficient speakers of English were asked to interpret bi-clausal multiple "wh"-questions in English (like "Where did the professor say the students studied when?"). Their responses were compared with those of a native speaker control group. It is argued that the results are consistent with the unavailability of the uninterpretable feature. Two conclusions are drawn: first, that there is a critical period for the selection of uninterpretable syntactic features for the construction of mental grammars; second, that despite the observation of target-like performance by L1 Japanese speakers on English "wh"-interrogatives reported in a number of existing studies, caution is required in interpreting target-like performance as evidence that L2 speakers have the same underlying grammatical representations as native speakers.