Usually water utilities provide their services under natural monopoly, with few incentives to become efficient, therefore affecting customers in the form of expensive tariffs. Hence, it is extremely important to find out the sources of inefficiency. The present study aims to identify the most efficient water utility groups in Brazil. For this purpose, a robust non-parametric method was applied. The results show that the utilities that provide both drinking water and wastewater services, the local utilities, and the utilities with private participation are more efficient. Furthermore, this study proved that the utilities were more efficient before the regulatory framework had been implemented.