Long-term exposure to a particular light regime or background – and therefore maintaining the chromatophores with a CI 1 or CI 5 – results in a slow change in the number of chromatophores and/or the amount of pigment, i.e. a morphological colour change, according to Babak’s Law (Babak, 1913). Our results give a clear-cut indication that such a morphological colour change takes place in Antarctic krill. Both, the concentration of the major pigment astaxanthin and the number of chromatophores in krill, are substantially higher in summer – when exposure to light is maximal – than in the darkness of winter.