1. Washed cell suspensions of Chlorobium thiosulfatophilium form large amounts of a polyglucose in the light. Addition of acetate to the cells increases the formation of polysaccharide considerable. During incubation in the dark, polysaccharide decreases with time, and organic acids such as succinic and propionic acid are excreted into the medium. 2. Glucose isolated from cells which had photoassimilated 1-, 2-, and U-14C-acetate had a specific activity which lay between 1 and 2 times that of the acetate substrates. 3. To analyse the distribution of radioactivity in the glucose units formed during photoassimilation of 14C-acetate, 2 microbial degradations, with bakers' yeast and Zymomonas mobilis respectively, were used. The results show that acetate gives rise to carbon atoms 1 plus 2 and 5 plus 6 of glucose, wheras carbon atomes 3 plus 4 are not labelled. Further, the results indicate that glucose is not formed via the reductive pentose phosphate cycle when acetate is present.