Abstract The present study compares levels of fluoride ion (F-) in the regulated Duratón River receiving an industrial effluent with safe concentrations (SCs) of F- for brown trout and rainbow trout to evaluate the responsability of fluoride pollution for the absence of trout populations downstream from the effluent. SCs for each trout species were estimated using the multifactor probit analysis software on acute mortality data. Differential releases from the dam caused short-term flow fluctuations at S-1 (between dam and effluent), S-2, S-3 and S-4 sampling sites (0.1, 2.2 and 7.3 km downstream from the effluent). Because of this, fluoride concentrations exhibited a temporal variation over a one-day period downstream from the industrial effluent; the highest and lowest concentrations (ppm F-) were 0.11 and 0.10 at S-1, 19.6 and 1.67 at S-2, 7.02 and 0.43 at S-3, and 2.98 and 0.52 at S-4. The mean F- concentration at the industrial effluent was 25.3±3.9 ppm. SCs (infinite hours LC0.01s) of F- for rainbow trout and brown trout were respectively 5.14 and 7.49 ppm. Comparisons between levels of F- in the Duratón River and SCs for trout species apparently indicate that fluoride pollution was a minor factor in determining the absence of trout populations downstream from the industrial effluent. It is concluded that intensive (24 hours) sampling surveys of pollutant levels should be undertaken in regulated rivers with pollution sources (e.g., industrial effluents) to evaluate appropriately the real influence of pollutants on freshwater organisms.