The speciation of iron (Fe) strongly influences the deposition and accumulation on gills causing toxicitytoward fish. The impacts of ferric (Fe(III)) and ferrous (Fe(II)) species on gill accumulation were studied in parallel flow-through channel experiments where Atlantic salmon (Salmo salar L.) was kept in cages. Downstream of the pH 6.3 mixing point, where Fe(III) ions or Fe(ll) ions were added continuously to lake water, the molecular mass of Fe(III) increased within 0.5 min after mixing due to hydrolysis and polymerization, while the Fe(II) species remained as low molecular mass (LMM) species 20 min after mixing. For fish exposed to the Fe(lll) enriched water (0.5 mg L(-1)) the Fe accumulation on gills was high and decreased downstream, while low when Fe(II) was added to water. By adjusting the Fe(II) enriched water to pH 6.7, the oxidation of Fe(II) forming Fe(III) accelerated, the Fe accumulation on fish gills increased by a factor of 3, and high mortality (33%) was observed. Thus, input of Fe(ll) ions, oxidation of Fe(ll) at rates higher than 1.5 microg L(-1) min(-1), and continuous formation of LMM Fe(III) species accumulating on gills can induce toxicity toward fish present in circumneutral freshwaters a long distance downstream from the entry points.