In the wild-type, colorless mutant cells (No. 9.80) of Chlorella kessleri (Fott et Novákóva, 211-11h) grown with glucose as a sole carbon source in the dark, the uptake of hexose (uptake of glucose, fructose or galactose) was reduced by up to 50% by a 30-min irradiation with white light.  This photo-inhibition was not observed under anaerobic conditions.  Colorless mutant cells contained Chl a equivalent to about 1% of that in the thylakoid membranes isolated from autotrophically grown wild-type cells and a trace amount of chlorophyllide (Chlide) a, but neither Chl b nor carotenoids.  Irradiation of the colorless mutant cells with white light for 30 min resulted in about a 80% decrease in absorbance at 673 nm.  The photobleaching of Chl a and Chlide a was partly suppressed under anaerobic conditions.  On the contrary, the photoinhibition of hexose uptake was not observed in a yellow mutant (211-11h/20) of Chlorella kessleri.  Although it lacked Chl, this yellow mutant contained normal carotenoids, which were not bleached by irradiation for 30 min. The addition of 40 µ M paraquat (methyl viologen), which produces superoxide largely inhibited the uptake of hexose in the colorless mutant.  Thus the photo-inhibition of hexose uptake observed in the Chlorella cells is assumed to be caused by active oxygen produced in the process of photobleaching of Chl(lide) a.