Acetate uptake by strains of Synechococcus and Aphanocapsa in short experiments required light, and was strongly inhibited by m-dichlorocarbonyl cyanide phenylhydrazone and dichlorophenyl dimethyl urea. Acetate carbon was distributed in amino acids and in the acyl portion of lipids in the same way as during growth experiments when CO2 was available, but the reduced incorporation in the absence of CO2 was primarily into the lipid fraction. An apparent K m for uptake by Synechococcus and for Aphanocapsa 6308 of 20 and 180 μM at pH 7.4 was obtained; corresponding V max values were 6 and 11 nmol x min-1 x mg protein-1. Uptake with Synechococcus was affected by pH, with affinity decreased and maximal rate increase with rising pH. Acetate uptake was not affected by propionate or butyrate when both were added at the same time, but a light and concentration dependent inhibition developed if suspensions were preincubated with propionate. Acetate carbon moved rapidly into acid insoluble material, but after 10–15 s 75% or more of the recovered intracellular counts were in acetyl CoA. Counts in this compound were reduced by preincubation with propionate. Kinetic measurements of acetyl CoA synthetase in fractionated cell extracts gave values for K m of about 50 μM for acetate, 5 mM for propionate, 100 μM for CoA and 0.38 mM for ATP. The internal pool of free CoA was measured to be about 20 μM, and was reduced by preincubation with propionate. This suggests that the activity of CoA-mediated reactions may be regulated by the availability of this cofactor.