The present study focuses on the effect of temperature on Al-chemistry and the subsequent toxicity to smoltifying Atlantic salmon, Salmo salar. Fish were exposed to acidic Al-rich water at different temperatures, and mortality, ventilation frequency and various blood parameters were measured. The relationship between temperature and Al-toxicity was documented as mortality increased systematically with increasing temperature. Physiological disturbances at the gills reflected the temperature dependent Al-toxicity. The temperature dependent toxicity observed is probably due to the degree of ongoing Al-polymerization.