Formal and informal institutions are closely linked and greatly depend on each other. As in other countries, Tanzania recently engaged in a far-reaching formal institutional reform towards Integrated Water Resources Management. This paper focuses on the interfaces and linkages between formal and informal institutional frameworks for water management in Tanzania with a case study of the Mkoji sub catchment in the Rufiji Basin. The paper identifies four major areas of interfaces, namely; centralized and local institutions; modern water rights and customary rights; Water User Associations and informal associations of water users; and formal and informal power relations. Keywords: Institutions, irrigation, water right, Tanzania.