This study assessed the prediction that individuals are able to use the knowledge from their first language (L1) in processing the comparable sound contrasts in an unknown language. Two languages, Arabic and Japanese, which utilize vowel duration contrastively, were examined. Native Arabic (NA) and native Japanese (NJ) listeners' discrimination accuracy for native (known) and nonnative (unknown) vowel length contrasts was assessed in an AXB discrimination test. A group of Australian English (OZ) speakers who do not know either Arabic or Japanese participated as a control group. Despite the expectation that native listeners positively transfer and generalize the L1 knowledge to process unknown languages with equivalent phonetic characteristics, both the NA and NJ groups were clearly less accurate in discriminating vowel length contrasts in unknown languages. Further, they showed no advantage over the OZ listeners who have limited experience with vowel length contrasts in their L1. These results suggest that, not only for stop place contrasts examined previously, but also for vowel length contrasts, experience with specific phonetic contrasts may not be sufficient for attaining truly nativelike discrimination accuracy.