Evidence is presented that a green photosynthetic bacterium (Chlorobium thiosulfatophilum, Tassajara) lacks ribulose 1,5-diphosphate carboxylase, the key enzyme of the reductive pentose phosphate cycle—the photosynthetic carbon reduction mechanism characteristic of green plants. The bacterium appears to use exclusively the reductive carboxylic acid cycle (and its associated reactions) not only, as previously recognized, in the photosynthetic conversion of CO2 to amino acids and organic acids but also to carbohydrates. This conclusion is based on: (i) the absence in cell-free preparations of ribulose 1,5-diphosphate carboxylase; (ii) the assimilation by cell suspensions of 14CO2, [14C]acetate, and [14C]succinate to give the products expected from the operation of the reductive carboxylic acid cycle; (iii) the demonstration in cell-free preparations of all enzymes needed for the conversion of CO2 and the primary product of the reductive carboxylic acid cycle, acetyl-CoA, to carbohydrate.