This paper analyzes two generations of Japanese speakers’ productions of [twi] and [ti] as they occur in native Japanese words and in loan words. Analysis across speakers verifies that this contrast is neutralized in native Japanese words and preserved in loans. Analysis of generational differences reveals two distinct patterns of preservation. Generation one speakers in this study produced overlapping distributions of [twi] and [ti]. In contrast, generation two speakers distinguished these strings in all environments. These data are consistent with the view that the first generation of borrowers mapped the foreign phonological contrast to an allophonic distinction in native Japanese and that the second generation of speakers promoted this weak phonetic distinction to phonemic status.