Abstract The effect of different concentration of K + and Na + on the skin colour of red sea bream and on melanosome movements in melanophores was investigated by using a video microscope system and subsequent image analysis. Soaking in artificial sea water containing 470 mmol/L NaCl, 10 mmol/L KCl, 10 mmol/L CaCl 2 , and 50 mmol/L MgCl 2 failed to prevent skin darkening, while solutions containing over 300 mmol/L KCl instead of the corresponding amount of NaCl induced aggregation of melanosomes, effectively preventing skin darkening. However, soaking in a solution containing 480 mmol/L KCl diminished the red colour of the fish skin. It is concluded that the solution containing 300 mmol/L KCl is useful to maintain skin colour of red sea bream and improve its commercial value.