Short-term static bioassays were conducted in duplicate to determine the toxicity of fluoride ion (F-) in soft water (hardness average value of 22 ppm CaCO3) to fingerlings (two months old) of Oncorhynchus mykiss Richardson and Salmo trutta fario Linnaeus. Animals were exposed to five different concentrations of sodium fluoride (NaF) for 8 days. They were also exposed to a high concentration of sodium chloride (NaCl) (800 ppm for O. mykiss and 1,000 ppm for S. trutta fario) as sodium and conductivity toxicity controls. No significant effect was obseved during NaCl tests, indicating that toxic effects generated by NaF were fundamentally due to fluoride ion. Fish in fluoride aquaria showed hypoexcitability, darkened backs and a decrease in respiration before their death. The 120, 144, 168 and 192 hour LC50 values were respectively 92.4, 85.1, 73.4 and 64.1 ppm F- for rainbow trout and 135.6, 118.5, 105.1 and 97.5 ppm F- for brown trout. O. mykiss appears significantly (P <0.05) more sensitive to fluoride ion than S. trutta fario. © 1991.