Scholars typically study affluent neighbourhoods resisting annexation by poorer adjacent cities. This paper focuses on the mirror image of this problem: municipal underbounding—the unwillingness of cities to annex poor neighbouring areas. In the paper, such local governments are called reluctant cities and it is suggested that urban studies scholars need to reach a better understanding of the practice. Here, a seemingly counter-intuitive case from California is documented where adjacent cities were convinced to annex poor neighbourhoods, designated as colonias. It is suggested that there may be opportunities for local co-operation leading to annexation. The important role of federal infrastructure funding is noted and the need for deeper involvement of residents of annexed neighbourhoods in decision-making is emphasised. Although race was not an issue in this case study, it is likely to be an important concern in annexations and the literature’s call for regional approaches and institutional reforms is supported.