An acetate kinase from the photolithoautotrophically grown purple bacterium Rhodopseudomonas palustris was purified to apparent homogeneity by use of high resolving liquid chromatography steps. The monomeric enzyme was characterized by a relative molecular mass of 46,500 and an isoelectric point of 4.9. There was an absolute requirement for divalent metal ions in the enzymatic reaction. Mg2+ and Mn2+ were the most activating cations. The acetate kinase used pyrimidine and purine nucleotides almost equally well as phosphoryl donors. The enzyme phosphorylated acetate, propionate, butyrate and isobutyrate. ATP and acetate revealed the lowest apparent Km values and seemed to act as the favoured substrates. The apparent Km values for ATP formation were considerable lower than those for the formation of acetyl phosphate. The activation energy Ea = 21 kJ/mol of the acetyl phosphate formation was determined by application of Arrhenius plots.