These experiments investigate whether the perceptual boundary between [a] and [a:] is affected by the differing phonotactics of the Sino-Japanese and Foreign strata of the Japanese lexicon. We presented a range of edited natural vowels from [a] to [a:] at the end of carrier nonwords of the form [CoC'__] and asked subjects to judge whether the final vowel was long or short, while C and C' were varied to make 9 carriers ranging from very Sino-Japanese-like to very Foreign-like. The perceptual boundary between [a] and [a:] was affected by both C and C', shifting through about 20 msec—a larger and more robust effect than was obtained in a word-superiority experiment with the same paradigm and subjects. These results cast doubt on explanations of phonotactic effects based on lexical-activation spreading. The influence of C at a distance of three phonemes from the ambiguous segment cannot be explained by simple segment-to-segment transitional probabilities.