Chromatic adaptation is the process by which blue-green algae alter the rates of biliprotein synthesis in response to changes in the color of available light. We have examined the control of phycoerythrin synthesis during the early stages of chromatic adaptation in Fremyella diplosiphon using fluorescence spectroscopy and (35)S-labeling of polypeptides. Phycoerythrin synthesis begins within 45 to 90 minutes after transfer of cells from red to green light, but is blocked by rifamycin. Transfer of cells from green to red light stops phycoerythrin synthesis with a t(1/2) = 45 minutes, as does the addition of rifamycin in green light. Transfer from green light to darkness slows but does not stop phycoerythrin synthesis. Gel electrophoresis of labeled polypeptides, both soluble and membrane-bound, shows that the synthesis of some polypeptides other than phycoerythrin are also affected by changes in light. These data suggest that chromatic adaptation involves gene regulation at the transcriptional level.