Background/Aims: Many studies have shown that experienced second language (L2) learners are more skilled than novice L2 learners at a variety of L2 tasks, including auditory discrimination between members of L2 phonological contrasts. In this paper we argued that while L2 experience is typically beneficial when comparing the effects of more versus less experience, it is not necessarily beneficial when comparing the effects of some experience versus none. Methods: We compared the perceptual assimilation and discrimination of the Korean sibilant fricatives /sh/ and /s*/ by 3 native Mandarin populations: naïve listeners, novice L2 learners (4-6 weeks of experience), and advanced L2 learners (over 2 years of experience). Results: The perceptual assimilation of /sh/ changed as a result of L2 experience, but only in the /a/ context. It is also shown that novice L2 learners were less accurate than the naïve listeners at discriminating between /sh/ and /s*/ but, crucially, only in the /a/ context. Conclusion: The perception of /sh/ by L2 learners may be affected by knowledge of the L2 unavailable to naïve listeners, and some aspects of this knowledge may result in a decline in discrimination accuracy after even a very short period of L2 experience.