Abstract Fluorescence induction m easurem ents indicate a PS IIα,β heterogeneity in the unicellular green alga Chlorella fusca. Under normal light the relative amount of PSα II is decreasing from 63% to 43% during the first 8 h of the cell cycle. The rate constant kα shows a concomitant decrease by 20% in the first 4 h. The relative amount and antenna size of PS IIα is restored in the dark. In contrast, PS IIβ - centres show the same rate constant during the whole cell cycle. Under low light kα is increased by approximately 30% over the cell cycle, whereas no change of kβ is observed. It is concluded that the PS II organization of Chlorella is changed both during the cell cycle and the adaptation to light intensity by a complex regulation (i) involving the antenna size of α-centres and (ii) changing the quantitative relationship of fast and slow reaction centres.