Whereas Labov (1991) made a case for the existence of three major dialects of English, this article offers Canadian evience that runs counter to the idea of a relatively homogeneous North American third dialect area in which vowel systems remain fairly stable. It shows that the lax vowels of Canadian English are undergoing a substantial shift, the pivot for which is suggested to be vowel merger in the Cot/Caught sets. This shift is to some degree conditioned by the voicing properties and the manner of articulation of a following consonant; gender differences prove significant as well. The article also examines back vowel fronting in Candian English and its relationship to the shift affecting the front lax vowels, as well as to the general principles of vowel chain shifting articulated by Labov (1994). The Canadian Shift raises the issue of internal versus external motivation of vowel change; in addition, it brings, macrosociolinguistic evidence to bear on the purely microsociolinguistic interpretation of similar patterns of vowel shifting as symbols of local group identity (Eckert, 1991b).