Directional asymmetries in cross-linguistic vowel discrimination, in which a change from a less peripheral vowel to a more peripheral vowel was found easier to detect than the reverse direction, has been well documented. However, the perceptual processes underlying the phenomenon remain to be fully understood. This study explored asymmetries in Mandarin lexical tone discrimination by native Mandarin and native Cantonese listeners. Both groups of listeners found a change from Mandarin tone 1 to all other Mandarin tones easier to detect than the reverse direction. However, the opposite was true for Mandarin tone 3. Neither processing load nor training had an impact on the results. Stimulus dynamicity, phonological underspecification and language-independent acoustic salience were discussed as possibly to account for the observed asymmetric pattern.