This study investigated the ability to discriminate the middle and low tone contrasts in Thai by two groups of native English (NE) speakers and a control group of native Thai (NT) speakers. The first group was comprised of NE speakers who had no prior experience with Thai, whereas subjects in the second group were experienced learners of Thai (EE). The variables under investigation were experience with Thai, discrimination of open versus closed syllables, and the interstimulus interval (ISI) of the presentation (500 vs 1500 ms). The results obtained indicated that the NT group obtained higher discrimination scores than the NE or EE groups, the EE group obtained higher discrimination scores than the NE group, all three groups of subjects found open syllables to be more difficult to discriminate than closed syllables, and subjects in the EE group obtained higher discrimination scores for open syllables in the shorter than the longer ISI condition.