This study investigated the presence or absence of two new functional features at a later stage of second language (L2) acquisition by Korean learners of English. It tested the predictions of the Failed Functional Features Hypothesis, which says that new features of functional categories in a second language are not acquirable beyond the critical period, assuming full transfer of a first language and partial access to universal grammar in post-puberty. The study examined the acquisition of a new functional category and its related features in the domain of determiner phrase (DP) in an advanced stage of interlanguage development. Two Korean post-critical-period graduate students with advanced English proficiency participated in a grammaticality judgment task, and their performances were compared with that of an English native-speaking control. Results suggested that functional features present in the L1, but not the L2, were absent in advanced learners' interlanguage. Implications of these findings for pedagogy are discussed in terms of provision of focused, explicit input, opportunities for output production, and timely feedback. The grammaticality judgment task is appended. (Contains 11 references.) (SM)