The Indonesian spoken on a daily basis in Jakarta, the national capital, shows considerable divergence from Standard Indonesian as it has been codified by the Centre of Language Development. A regional dialect appears to be developing, resulting from contact between superposed Standard Indonesian and a number of regional varieties. This paper documents one part of this development, the interaction of Standard Indonesian and the native Jakartan variety, Betawi. An analysis of verb morphology, and the effect of register shift on its use, show that a process of koineization has taken place, and that Jakarta Indonesian is crystallizing into a separate variety of Indonesian, a nativized koine, with fixed norms of usage.