ABSTRACT Service level of water supply systems (WSS) was assessed across in Ogun State, Southwest Nigeria. The WSS included: self-supply systems, communal water supply systems, and public water systems. Five service level indicators (access, quality, management, cost, and reliability) were assessed using structured questionnaires, targeting WSS users and owners in two categories: owners/resident users and non-resident users. Results show that owners/resident users restricted access of non-resident users to water assets, hence reducing adequate service level. Post-collection treatment was practiced by some users, using sodium hypochlorite solutions. Management and costs of WSS are borne by owners/resident users; costs borne by non-resident users is only from purchase of water. Reliability of WSS is hampered by geology and climatic seasonal variations as some WSS dried up during the off-rain season. Poor access to water has been identified as a major challenge facing non-resident users, amplified by limited coverage area of public water supply systems.