In crude extracts ofChlorella kessleriFott and Novákóva cells grown autotrophically in white light the activity of phosphofructokinase (PFK, EC 2.7.1.11) is 62.9 ± 1.5 nmol (mg protein)−1min−1under optimized test conditions. It is greatly increased in red [88.3 ± 1.8 nmol (mg protein)−1min−1], but somewhat decreased [57.0 ± 0.5 nmol (mg protein)−1min−1] in blue light of equal productivity. Mixtures of blue and red light yield the low activity as long as blue light represents at least 35% of the total quantum fluence rate. The rough wavelength dependence of the counteracting effect of short wavelength light on the increasing effect of red light exhibits a broad peak at 460 nm, reminiscent of action spectra of the blue/UV photoreceptors(s). Upon transfer of red light‐grown cells to blue light, the decrease develops slowly within 72 h; it cannot be prevented by 3‐(3,4‐dichlorophenyl)‐1,1‐dimethyl urea (DCMU). Since there is less carbohydrate in blue than in red light‐exposed cells, correlations between biosynthesis of PFK and level of carbohydrate are discussed, based on the assumption that red light decreases and/or blue light increases the transport of metabolites acr