The research presented in this thesis comprises two experiments which investigated whether the domain of phonetics can undergo first language attrition, or be lost, when a second language is acquired in adulthood in a migrant context. Experiment I investigated the native speech of 57 German migrants to Anglophone Canada and the Dutch Netherlands. The bilingual migrants had grown up in a monolingual German environment and moved abroad in adolescence or adulthood. Their semispontaneous German speech was globally assessed for foreign accent by native German speakers in Germany. It was revealed that 14 bilingual migrants were perceived to be non-native speakers of German. Age of arrival to Canada or the Netherlands and contact with one’s native language played the most significant roles in determining whether the German speech of the migrants was assessed to be foreign accented. Crucially, it was not only the amount of contact, but also the type of contact which influenced foreign accented native speech. Monolingual settings, in which little language mixing was assumed to occur, were most conducive to maintaining non-foreign accented native German speech. These findings prompted Experiment II, in which the speech of 10 German migrants to Anglophone Canada was examined in fine phonetic detail. The participants in this experiment had similarly grown up in a German speaking environment and migrated to Canada in late adolescence or adulthood. Segmental and prosodic elements of speech, which generally differ between German and English, were selected for acoustic analyses. Given that each phonetic element was measured according to two dimensions, it was possible to determine that in the lateral phoneme /l/, the frequency of F1 was more likely to evidence first language attrition than the frequency of F2; and that in the prenuclear rise, the alignment of the start of the rise was more likely to display first language attrition than the alignment of the end. In addition to intrapersonal variation within the same phonetic variable, interpersonal variation was observed. Two participants evidenced no first language attrition, whilst one participant realised both dimensions of the lateral phoneme /l/ and prenuclear tonal alignment according to the Englishmonolingual norm in his German. When extralinguistic variables were investigated, age of arrival (and neither amount nor type of language contact) had a significant impact on determining first language attrition, although this effect was only observed in the alignment of the prenuclear rise. While the experiments revealed stability in the native speech of late consecutive bilingual migrants, first language attrition in the domain of phonetics was observed at both the level of perception and performance. Taken together, these findings challenge the traditional concept of native speech by revealing that indeed native speakers diverge from the norms of native (monolingual) speech.