ABSTRAKThis study deals with the acquisition of Indonesian varieties used in Jakarta? the standard variety of Indonesian, the Bahasa Indonesia (BI) and the non-standard one, the Colloquial Jakarta Indonesian (CJI) ? by children of middle class families. Many today?s Jakarta Indonesian children learn Indonesian as their first language in informal settings, and generally they learn the informal variety first, namely Colloquial Jakarta Indonesian (CJI) (see also Wouk 1989, 1999). Yet, at very early age, before formal schooling, they are also confronted with the formal variety of Indonesian, the Bahasa Indonesia (BI) as they learned from children program in television or story-telling by parents. Children are also encouraged to speak BI as a means of politeness.The focus in this study is on factors that influence choice of the language variety, capability of style-shifting, and morphological variation. The main research questions concern the following.