The acetate metabolism of autotrophically grown, acetate-adapted and acetate-grown cells is compared. All oxidize acetate rapidly and assimilate about half of the acetate added in short-term experiments. Kinetic analysis of the incorporation of 14C-acetate reveals citrate as a primary product of acetate assimilation in all cells in darkness. Malate formed from acetate-I-14C by acetate-grown cells is asymmetrically labelled in a manner consistent with a primary incorporation of acetate into malate by a malate synthase reaction. The chief difference between autotrophic and acetate-grown cells is the faster rate at which the latter incorporate acetate carbon into compounds outside the tricarboxylic acid cycle. In particular, incorporation into protein and carbohydrate is much faster in acetate-grown cells and it is suggested that enzymes catalysing reactions leading away from the tricarboxylic acid cycle may increase in activity in acetate-grown cells. Light greatly stimulates acetate incorporation into lipide and also increases the synthesis of protein and carbohydrate.