Objective: To evaluate fluoride concentration in public water supply in locations of Maranhão State that, according to agency responsible, presents regular water fluoridation. Methods: Water samples were collected from public schools. According to National Health Foundation (FUNASA), Water Treatment Station (WTS) of 14 rural cities and 03 WTS from Sao Luiz are supplied by the water supply system of state. The fluoride content of the analysis was performed using a specific electrode for fluoride. Three samples from each school were collected, totaling 42 samples of the rural cities and 45 samples from the capital. Results: Most samples were below the optimal concentration, and only 14.28% of samples collected in the municipalities and 6.67% in Sao Luiz, had acceptable values of fluoride (0.6 to 0.8 ppmF). Conclusion: Fluoridation of public water supplies (FPWS) is uncontrolled and therefore no preventive effect on caries. It is important to emphasize the need to implement control measures to ensure the ongoing effectiveness of FPWS in Maranhão State.