Through all periods of Gaulish, viz. Early, Middle and Late Gaulish, the sibilants is amply attested in word-initial, word-medial and word-final po-sition in the vernacular language material. Therefore it is inadmissible to assume a general rule of "lenition" of s (i.e. aspiration or loss). A close ex-amination of the material reveals, however, that a set of very specific rules of s-deletion operated on the language: first, loss of sin word-initial posi-tion in proclitics; secondly, loss of s in the onset of second syllables when the first syllable also started with s; thirdly, assimilation of s to resonants. These three rules probably operated at the Common Celtic stage. A fourth rule, the optional weakening and loss of word-finals affected Gaulish from the Middle Gaulish period onwards and is a development not shared by Vulgar Latin of the time.