Cross-nationally, urbanization is associated with the decline of minority languages and a shift towards national and official languages. But the processes that link urbanization with language shift have not been adequately documented. In this paper we consider the relationship between cities and language shift from a sociolinguistic perspective, focusing our attention on the issue of language use and language shift in Indonesia -- a large, ethnically and linguistically diverse, rapidly urbanising country. We use census data to examine how ethnic diversity shapes language shift in the context of urbanicity. We find that in ethnically homogenous regions, urbanicity itself has little relationship with language shift. By contrast, ethnic diversity is consistently associated with a greater probability of speaking Indonesian both among urban and rural Indonesians and in urban and rural areas. These findings contribute to our understanding of language shift and linguistic vitality in diverse, urbanising societies, and highlight the need to distinguish between the process of urbanization and the state of being urban.