This paper identifies the main dialects within Nyamwezi, a Bantu language of Tanzania, and clarifies the historical relationship between these Nyamwezi (F.22) dialects and Sukuma (F.21). I claim, contrary to the conventional wisdom regarding these languages, that a rough linguistic border exists, which separates the Nyamwezi varieties from Sukuma. By implication, Sukuma and Nyamwezi do not exist in a dialect continuum with one another, and the Ndala lect described in Maganga and Schadeberg (1992) should be considered Sukuma. These claims are supported by primarily lexical and phonological evidence gathered during recent surveys conducted by SIL International. Furthermore, Batibo's (2000) relative chronology of the main innovations considered in this present study (*c/*j fricativization, Bantu spirantization, Dahl's Law, and *p-lenition) is re-examined in light of this new evidence. This paper demonstrates how diachronic dialectology can shed light on the dualistic processes of divergence and convergence in Bantu, and the resulting spread of linguistic innovation.