In many cities of the developing world, dynamic informal infrastructure services work side by side with formal infrastructure services. Informal urban water service is almost as important as formal water supply in providing adequate water for urban inhabitants. This research tries to contribute to our understanding of informal infrastructure services in cities of developing world, using urban water supply in Metropolitan Jakarta as a case study. An indigenous system of urban water vendors operation, the so-called Sistem Aplus, allows vendors to work in a city while still maintaining their residencies in a village. This fact explains the circular migration phenomena in Jakarta, where temporary farmers in rural areas work and live partly in Jakarta. Informal water vendors operate in a laissez-faire system. They strategize their operations based on markets and consumer behavior. Self-regulation and self-policing among water vendors is the effective means of promoting efficiency and safety within the informal water deliveries. In devising the appropriate regulatory framework, it is important to consider the social and political contexts in which water-supply market will operate.