This study examined the perception of short and long vowels in Arabic and Japanese by native (NJ) and non-native (NNJ) speakers of Japanese. Both Arabic and Japanese use duration contrastively. The aim was to examine whether and how the listeners' differential knowledge of Japanese might influence the processing of short and long vowels in an unknown language, Arabic, as well as Japanese. Of particular interest was the effect of NNJ's emerging length categories resulting from second/foreign language learning experience on their cross-language perception. Ten Japanese vowels (five short, five long) were used as response categories. On average, the NJ listeners categorized the Japanese vowels more accurately than did the NNJ listeners (91 vs. 84%), in particular, long vowels (94% for NJ and 82% for NNJ), but the between-group difference did not reach statistical significance. However, qualitative differences were observed between the two groups in the way they responded to Arabic and Japanese vowels. This may be related to the distributional characteristics of Japanese vowels, which is that long vowels are much less frequent than short vowels, prompting the NNJ, but not the NJ, listeners to select the short category over the long counterpart when in doubt. As a result of this distributional imbalance, the NNJ listeners may have developed a more robust representation for short than for long vowels in Japanese. Both NJ and NNJ listeners assimilated Arabic vowels to a wide range of Japanese vowels, suggesting perceptual confusion.