Perceptual training was employed to characterize individual differences in non-native speech sound learning. Fifty-nine adult English speakers were trained to distinguish the Hindi dental-retroflex contrast, as well as a tonal pitch contrast. Training resulted in overall group improvement in the ability to identify and to discriminate the phonetic and the tonal contrasts, but there were considerable individual differences in performance. A category boundary effect during the post-training discrimination of the Hindi but not of the tonal contrast suggests different learning mechanisms for these two stimulus types. Specifically, our results suggest that successful learning of the speech sounds involves the formation of a long-term memory category representation for the new speech sound. (Contains 1 table and 10 figures.)