In three separate experiments using controlled natural stimuli and a high-amplitude sucking paradigm, infants' ability to detect differences between /s/ and /v/, /s/ and /∫/, and /s/ and /z/, respectively, was investigated. Evidence for discrimination was obtained for /s/ versus /v/ and /s/ versus /∫/ but not for /s/ versus /z/. Implications for a theory of infant speech perception are discussed.