Mintzberg s five different structural configurations for organizations is a useful tool for both classifying and identifying evolutionary trends in organizations. His model was applied to make a critical analysis of the structure adopted by the Brazilian Water Authority - ANA. ANA was created by Law 9984/2000 with the mission of regulate the use of a public resource - the water of the rivers - and to implement the National Water Resources Management Program. Thus, ANA has a double personality aspect and must act both as a regulatory agency and as policy maker for the Water Sector. Based on a compilation of institutional and legal documents and also on field research, important data was collected to: (i) identify ANA s mission and functional design; (ii) verify if the structural organization met the functional requirements; (iii) study alternatives for the organizational architecture, using Mintzberg s theory. It was found that ANA is currently in an initial organizational stage, similar to the simple structure of the taxonomy proposed by Mintzberg. However, a quantitative accumulation of deficiencies in this initial configuration has been observed, thus foreseeing a qualitative shift towards more sophisticated structures. In particular, it was observed that ANA s mission would be better met if a hybrid structure of professional bureaucracy and adhocracy was adopted.