Previous research has found that a speaker's native phonological system has a great influence on perception of another language. In three experiments, we tested the perception and representation of Mandarin phonological contrasts by Guangzhou Cantonese speakers, and compared their performance to that of native Mandarin speakers. Despite their rich experience using Mandarin Chinese, the Cantonese speakers had problems distinguishing specific Mandarin segmental and tonal contrasts that do not exist in Guangzhou Cantonese. However, we found evidence that the subtle differences between two members of a contrast were nonetheless represented in the lexicon. We also found different processing patterns for non-native segmental vs. non-native tonal contrasts. The results provide substantial new information about the representation and processing of segmental and prosodic information by individuals listening to a closely-related, very well-learned, but still non-native language. (Contains 9 figures and 3 tables.)