Many varieties of Arabic and Berber have undergone a process of syntactic change known as Jespersen’s Cycle (JC). In JC a postverbal item is reanalysed as part of a discontinuous marker of negation together with the original preverbal negator. In some cases the original preverbal negator is then lost. This paper investigates the synchronic and historical data relevant to JC in the varieties in question, arguing that the innovation of the postverbal negator began in Arabic and later spread to Berber through contact. The various syntactic reanalyses involved in the Arabic JC are sketched, and the implications of the Arabic and Berber data for better-known instances of JC in European languages are discussed.