The objective of this study is to identify natural sources of fluoride ion contamination in groundwater in Guidan Roumji Department, Niger. Samples of drilling water trapping said sheet were taken and certain physicochemical parameters determined. The fluoride ion content of the water sampled varies from 0.49 to 2.17 mg / L and indicates that some of the borehole water in Guidan Roumji Department is unfit for consumption. The results of the correlations between fluoride ions and other ions show that the dissolution of hornblende is the main source of contamination by fluoride ions in groundwater, in the Department of Guidan Roumji in Niger.