Japanese has a vowel duration contrast as one component of its language-specific phonemic repertory to distinguish word meanings. It is not clear, however, how a sensitivity to vowel duration can develop in a linguistic context. In the present study, using the visual habituation-dishabituation method, the authors evaluated infants' abilities to discriminate Japanese long and short vowels embedded in two-syllable words (/mana/ vs. /ma:na/). The results revealed that 4-month-old Japanese infants (n = 32) failed to discriminate the contrast (p = 0.676), whereas 9.5-month-olds (n = 33) showed the discrimination ability (p = 0.014). The 7.5-month-olds did not show positive evidence to discriminate the contrast either when the edited stimuli were used (n = 33; p = 0.275) or when naturally uttered stimuli were used (n = 33; p = 0.189). By contrast, the 4-month-olds (n = 24) showed sensitivity to a vowel quality change (/mana/ vs. /mina/; p = 0.034). These results indicate that Japanese infants acquire sensitivity to long-short vowel contrasts between 7.5 and 9.5 months of age and that the developmental course of the phonemic category by the durational changes is different from that by the quality change. (Contains 1 table, 5 figures and 3 footnotes.)