The underlying premise of this study was that the two phonetic subsystems of a bilingual interact. The study tested the hypothesis that the vowels a bilingual produces in a second language (L2) may differ from vowels produced by monolingual native speakers of the L2 as the result of either of two mechanisms: phonetic category assimilation or phonetic category dissimilation. Earlier work revealed that native speakers of Italian identify English /eI/ tokens as instances of the Italian /e/ category even though English /eI/ is produced with more tongue movement than Italian /e/ is. Acoustic analyses in the present study examined /eI/s produced by four groups of Italian-English bilinguals who differed according to their age of arrival in Canada from Italy (early versus late) and frequency of continued Italian use (low-L1-use versus high-L1-use). Early bilinguals who seldom used Italian (Early-low) were found to produce English /eI/ with significantly more movement than native English speakers. However, both groups of late bilinguals (Late-low, Late-high) tended to produced /eI/ with less movement than NE speakers. The exaggerated movement in /eI/s produced by the Early-low group participants was attributed to the dissimilation of a phonetic category they formed for English /eI/ from Italian /e/. The undershoot of movement in /eI/s produced by late bilinguals, on the other hand, was attributed to their failure to establish a new category for English /eI/, which led to the merger of the phonetic properties of English /eI/ and Italian /e/. © 2002 Elsevier Science B.V. All rights reserved.