Attunement to native phonological categories and the specification of relevant phonological features in the lexicon occur early in development for monolingual and monodialectal speakers. However, few studies have investigated whether and how early exposure to two dialects of a language might influence the development of phonological categories, especially when a phonemic contrast exists only in one dialect. This study compared perceptual sensitivity to mispronunciations in phonemic vowel length in Australian English adult listeners with and without early exposure to another English dialect that did not have this contrast. The results showed that, while both mono- and bi-dialectal groups were sensitive to mispronunciations in vowel length, the bi-dialectal adults were more likely to accept a mispronunciation in vowel length compared to mono-dialectal adults. The bi-dialectal group accepted mispronunciations in vowel length more than in vowel height and backness. These results suggest that the bi-dialectal Australian English adults may employ a more flexible vowel length category for spoken word processing compared to mono-dialectal adults. The findings reveal a complex influence of early exposure to another dialect on the development of phonological categories.