A protein fraction (A-L fraction) with characteristics reminiscent of muscle actin has been isolated from Escherichia coli. The A-L fraction undergoes reversible aggregation under the same conditions in which actin is polymerized and depends primarily on potassium for its polymerization. This fraction, upon electrophoresis on acrylamide gels in the presence of sodium dodecyl sulfate, exhibits a distinct peak at the characteristic molecular weight of 45,000. Passage of skeletal muscle myosin through the A-L fraction specifically removes this 45,000-molecular weight peak. Examination of the myosin by sodium dodecyl sulfate electrophoresis after the passage reveals a new band at the proper molecular weight. The A-L fraction from wild-type E. coli is compared with the protein from a potassium transport mutant. Important catalytic differences exist between the A-L fractions of the two strains. The A-L fraction from the mutant fails to polymerize in low-K media in the K+ concentration range in which the mutant fails to take up to K+. In low-K+ media, the parent strain accumulates potassium and the A-L fraction from this organism polymerizes. The cell swelling reaction of both strains has been studied. Parent cells swell during low-K+ uptake, whereas the mutant does not. It is construed from this that the differences in the characterization of the A-L fraction relative to that of the wild type are related to the loss of cell swelling in the mutant and hence to the loss in alkali cation selectivity. The possible role of contractile proteins in biological ion exchange is discussed.