Recent years have seen a proliferation of research on attrition in L1 speech (de Leeuw, Mennen & Scobbie, in press; de Leeuw, Schmid & Mennen, 2010; Dmitrieva, Jongman & Sereno, 2010; Mennen, 2004). Adding to this line of inquiry, the present study investigates the speech of a 62-year-old bilingual monozygotic twin who emigrated to an L2-speaking environment 30 years ago. Changes in L1 accent were assessed by comparing her speech to that of her identical twin sister who remained in the L1-speaking environment, thus providing a unique control setting. Acoustic analyses of voice onset time and vowels indicate pervasive changes to the emigrated twin's L1 accent, with attrition presenting in the form of cross-linguistic assimilation patterns. Interestingly, her L1 vowel space exhibited a systematic increase in first formant frequency, confirming claims that L1 and L2 sounds may be related to each other at a system-wide level (Chang, 2010, 2011; Guion, 2003). Implications for theoretical models of bilingual sound systems are discussed.