A body of research has shown the effect of lexically guided perceptual learning (e.g., Norris et al., 2003), whereby listeners modify phonetic category boundaries after being exposed to critical words that contain segments that are phonetically ambiguous and serve as new exemplars of a target phonetic category. It has been shown that several factors constrain the extent of learning, and one recent study (Stevens et al., 2007) suggests that inherently variable phonetic categories might resist retuning. To test this idea, we examined modifiability of two vowel categories, [i] and [u], the latter of which has a wider range of variability than the former due to contextual variation as well as ongoing sound change. Our listeners heard exposure stimuli that were ambiguous between [i] and [u], embedded in lexical frames that yield real words only if the vowel is /i/ (/i/-words) or /u/ (/u/-words), during lexical decision tasks. Each listener’s category boundary between [i]-[u] was measured Pre- and Post-exposure. Our results show that listeners tend to shift boundaries more for [i] than [u], suggesting a way that listeners balance between flexibility and stability in speech perception.