This paper proposes a phonetically based account of phonological stop assibilation. We first review evidence for the phonological representation of sibilants proposed by Jakobson et al. (1951) and others. An examination of cross-linguistic data involving phonological assibilation reveals two asymmetries: (a) only high vocoids constitute assibilation triggers; (b) only following vocoids, not preceding vocoids, constitute assibilation triggers. It is then shown that high vocoid-conditioned phonological stop assibilation is motivated by the brief period of turbulence that sometimes occurs at the release of a plosive into a high vocoid. We argue that this turbulence is interpreted in phonology as the insertion of the feature [+strident] into the representation of the plosive, with the loss of the feature [−continuant] in the case of frication. Not only does this phonetic analysis explain the phonetic origin of assibilation, it also correctly reflects the cross-linguistic conditions on high vocoid-conditioned phonological assibilation.