The present study tested Japanese 4.5- and 10-month old infants' ability to discriminate three German vowel pairs, none of which are contrastive in Japanese, using a visual habituation-dishabituation paradigm. Japanese adults' discrimination of the same pairs was also tested. The results revealed that Japanese 4.5-month old infants discriminated the German /bu:k/-/by:k/ contrast, but they showed no evidence of discriminating the /bi:k/-/be:k/ or /bu:k/-/bo:k/ contrasts. Japanese 10-month old infants, on the other hand, discriminated the German /bi:k/-/be:k/ contrast, while they showed no evidence of discriminating the /bu:k/-/by:k/ or /bu:k/-/bo:k/ contrasts. Japanese adults, in contrast, were highly accurate in their discrimination of all of the pairs. The results indicate that discrimination of non-native contrasts is not always easy even for young infants, and that their ability to discriminate non-native contrasts can improve with age even when they receive no exposure to a language in which the given contrast is phonemic.