Listeners were presented with stimuli from a nine syllable [?a]–[da]continuum (constructed using a natural speech [?a]) in which syllables differed in the duration of frication from 52 to 8 msec. In each experimental session, an initial identification test was conducted to determine the category boundary. This base‐line test was followed by one of three adaptation tests in which subjects were presented with repeated syllables. The adapting stimuli consisted of (a) a single [?a]adaptor with 52 msec of frication, (b) a single [?a] adaptor with 32 msec of frication, (c) a ’’variable’’ adapting sequence that included the first six members of the test series and had a mean frication duration of 37 msec. The variable adapting sequence produced an adaptation effect equal in magnitude to the effect produced by the single adaptor with 32 msec of frication. The adapting syllable containing 52 msec of frication produced a significantly larger effect. The results demonstrate that (a) selective adaptation of speech did not require presentation of a constant adapting syllable and (b) the amount of adaptation depends upon the mean duration of frication contained in the adapting material at least up to 52 msec. A second experiment was performed to determine whether adaptation along a [?a]–[da] continuum was linearly dependent upon friction duration at values beyond 52 msec by comparing adaption produced by [?i], [?i], and [di]adaptors, containing friction duration of 120, 60, and 12 msec, respectively. The results showed that [?i] and [?i]produced highly similar adaptation effects on a [?a]–[da]series. The results suggest that an analyzer underlying perception of [?] is neither linearly nor symmetrically tuned to a particular range of friction duration.