A new cyanobacterial isolate, morphologically closely resembling Aphanocapsa, was characterized for its growth requirements, as well as pigmentation, photosynthetic activity and dynamics of the D1 protein in the reaction center (RC) of photosystem II (PSII). It was shown to be able to grow on glucose in the dark in the presence of DCMU. The cyanobacterium turned light yellow at high light intensity in the absence, and dark emerald green in high light in the presence of sublethal concentrations of the DCMU‐type inhibitor atrazine. While total carotenoids per cell slightly decreased with increasing light intensity during growth, the cells still turned pale yellow due to decreased levels of chlorophyll and phycocyanin. In contrast to β‐carotene, zeaxanthin and echinenone which decreased with increasing light intensity during growth, the carotenoid glycoside, myxoxanthophyll, continuously increased in concentration. Extremely high rates of light‐saturated O2 evolution were recorded for the high light cultures after a 0.5 h recovery period in the dark. The recovery measured after 2.5 h was shown to be less effective in darkness than in dim light and was prohibited by chloramphenicol. The degree of recovery was dependent on the light intensity during growth. A fast light intensity‐dependent RC II‐D1 protein turnover was found for the bleached yellow cells rich in myxoxanthophyll. The half‐life of the RC II‐D1 protein, plotted against the light intensity during growth and experimentation, yielded a curve the slope of which was considerably steeper for Aphanocapsa than for Anacystis. Apparently, the isolated strain of Aphanocapsa reacts more vigorously to changes in the environment than other strains tested and may, therefore, turn out to be a suitable organism in the attempt to elucidate the molecular mechanism of light intensity adaptation.