The uptake of 3-O-methyl-(14)C-D-glucose, a non-metabolizable sugar, by autotrophically grown Synechococcus cedrorum was studied at low sugar concentrations in the incubation medium (0.71-11.36 μM), in the light and in the dark. Optimum sugar accumulation against a concentration gradient occurred within dark-treated "starved" cells that were incubated in the light. This phenomenon was greatly inhibited by metabolic inhibitors; it was much less when sugar uptake was observed in the dark. Control cells incubated in the light accumulated 3-O-methyl-(14)C-D-glucose against a concentration gradient only at lower sugar concentrations (0.71-2.84 μM) and to a lesser extent than the dark-treated cells. Sugar uptake against a concentration gradient by the control cells was completely inhibited in the dark. The results indicate that the morphologically simple unicellular blue-green alga, S. cedrorum, is under certain conditions capable of obtaining a sugar from its medium by an active transport process.