AbstractNatural populations and culture strains ofOscillatoria redekeiisolated from the same lakes exhibit a great variety of phycoerythrine/phycocyanine ratios even under the same growth conditions. Cytophotometric and spectrophotometric determinations of the phycoerythrine and phycocyanine contents of different strains after exposure to blue‐green and orange‐red light, respectively, suggested that chromatic adaptation ofO. redekeitakes place both at cellular and population level. This wide intraspecific diversity is considered at least one reason for competitiveness in highly eutrophic la