From experiments with a subspecies of the Japanese fish Tribolodon hakoniensis it is proposed that the mechanism of chloride excretion cells which is part of the osmoregulatory system of fishes also provides a mechanism for acid tolerance through HCl excretion. Thus salt tolerant fresh water fish species are preselected for a degree of acid tolerance. Spawning habits that expos e eggs and fry to specially acid waters appear to be an important factor to render a species susceptible to acidification. The example of Tribolodon Hakoniensis shows that modification in spawning habits is one of the mechanisms for adaptation. A population of common carp in Aneboda, Sweden, that lived for some 15 generations in humic, acid water show higher acid tolerance than genetically related carp populations. They also show higher concentrations of red blood cells. It is proposed than an increase in red blood cells may be mechanism through which improvement in acid tolerance can be achieved in a fish population through selection within some several decades.