Consumption of unusually high concentrations of F(-) in groundwaters of the Maria area in the Gaspé peninsula of Quebec have resulted in symptoms of skeletal fluorosis in two members of the population. One of these individuals consumed approximately 50 mg of fluoride per day over a 6 year period before being hospitalized and later diagnosed with skeletal fluorosis. It is estimated that, until this case came to light, approximately 15-20% of the rural population (total approximately 1,600) in the area were consuming groundwaters with F(-) levels between 5 and 28 mg L(-1) for at least 6 years. The high concentrations of F(-) in well waters of the Maria area occur only in wells completed in Carboniferous sandstone-siltstone-conglomerate sediments that underlie a thick blanket of alluvial-colluvial-glacial overburden. These fluoriferous groundwaters exhibit high Na and HCO3 (-) contents and low Ca and Mg concentrations compared to those associated with the overburden sediments. The high F levels greatly increase the risk for fluorotic diseases such as skeletal fluorosis and skeletal radiculomyopathy. Wells completed in overburden, although having suboptimal F(-) levels are safer for the health of individuals in this region. Effective regulations for well drilling need to be formulated for regions underlain by Carboniferous formations in the Maritime provinces of Canada. In some regions, high F(-) levels (10-25 mg L(-1)) in groundwaters will seriously affect how, and to what extent, groundwater supplies can be developed for domestic use.