Many pollutants, even when present in the water in concentrations well below lethal levels, may cause marked changes in the physiology and behaviour of fish. The work reported deals mainly with salmonids. The responses to insecticides are particularly interesting. Of fundamental interest is the suggestion that DDT seems to act by interfering with the normal thermal acclimation mechanism(s), probably within the central nervous system. The results are not without practical significance. Some responses, including those induced by heavy metal ions from mining wastes, may cause long-term ecological changes of consequence.