In this study, we investigated language-specific developmental changes in Japanese infants' perceptual discrimination of English approximants, /r-l/ and /w-y/ at 6-12 months. It was found that both /r-l/ and /w-y/ were discriminated at 6-8 months. At 10-12 months, however, /w-y/ was discriminated, whereas /r-l/ was not. The present findings support the following conclusions; 1) a developmental decline in discrimination of non-native approximants occurs toward the end of the first year, consistent with earlier reports on non-native stop consonant contrasts; 2) phonemic factors are crucial in accounting for the observed decline, given that discrimination of a native approximant contrast remains good throughout the first year.