High levels of fluoride in freshwater ecosystems are harmful for humans and animals, particularly, aquatic biota. In this paper, the concentrations of fluoride in arsenic-rich lotic ecosystems of the Pampa Plain, Argentina, were recorded. The relationship of fluoride with the concentration of arsenic was explored. Our results show that fluoride in these freshwater ecosystems ranged from 0.15 to 1.65 mg L(-1). Concentrations of fluoride were highly and significantly (r=0.71; p=0.001) correlated with arsenic.