This study reports differential category retuning effect between [i] and [u]. Two groups of American listeners were exposed to ambiguous vowels ([i/u]) within words that index a phoneme /i/ (e.g., athl[i/u]t) (i-group) or /u/ (e.g., aftern[i/u]n) (u-group). Before and after the exposure these listeners categorized sounds from a [bip]-[bup] continuum. The i-group significantly increased /bip/ responses after exposure, but the u-group did not change their responses significantly. These results suggest that the way mental representation handles phonetic variation may influence malleability of each category, highlighting the complex relationship among distribution of sounds, their mental representation, and speech perception.