Inhibitors of the phytoene desaturase in carotene biosynthesis were tested in the enhanced rapid turnover of the D1 protein of photosystem II in high light exposure of Chlamydomonas reinhardtii cells. After 1 h high light on heterotrophically grown cells in the presence of norflurazon or fluridone, photosynthesis activity in vivo and PS II activity in vitro is lost. The D1 protein has disappeared. PS I activity is not affected, nor is the D2 protein. It is concluded that beta-carotene is essential for the assembly of the D1 protein into functional photosystem II. It is suspected that bleaching of beta-carotene in the reaction center of PS II by high light destabilizes the structure and triggers the degradation of the D1 protein.