Japanese loanwords from English exhibit many peculiar patterns with respect to the occurrence of geminate consonants. In this paper we focus on a positional asymmetry whereby coda consonants in the source words tend to be geminated in word-final position but not in word-medial position. Considering this phenomenon both from cross-linguistic and experimental perspectives, we first demonstrate that the positional asymmetry cannot be attributed to position per se but rather to some phonetic differences between word-final and non-final coda consonants in the source words. We then report perceptual and acoustic data to explore the phonetic features that trigger consonant gemination in loanwords. The perceptual data show that native Japanese listeners respond sensitively to differences in pitch when perceiving consonant length. In addition, the acoustic data demonstrate that final syllables in English are longer than non-final ones and are often produced with a noticeable pitch fall. These data suggest that Japanese listeners are sensitive to the phonetic differences between final and non-final coda consonants in the source words, thus showing a positional asymmetry in loanwords.