Among countries with river basin organizations to manage their water resources, Spain's experience is one of the longest. One of the first basin agencies established in Spain was for the Guadalquivir River in the south. A case study of that river basin and its management indicates how basin management is shaped by political economy factors such as the historical path of the agency's evolution, the basin agency's relationships with central government and with regional or local governments, the patterns of water user representation within the agency, and developments in water law and policy external to the basin agency. The case raises questions about whether and how integrated water resources management at the river basin scale is implemented, even in locations where basin agencies already exist. It also suggests that the politics of management at the river basin level will affect the implementation of national water policies intended to promote integrated management.