The adenosine triphosphatase (ATPase) activity in Escherichia coli cells was investigated by a combination of electron microscopic and cytochemical methods. The activity is located in the cytoplasm and cytoplasmic membrane. After prolonged incubation in the substrate medium, the insoluble calcium phosphate was concentrated into several conglomerations and probably extruded from the cell. The possible mechanism of extrusion is also discussed.The location of ATPase in the membrane preparations was investigated by similar technique. The activity in the membranes is located in between the membrane subunits. The order of the subunits became well arranged after incubation with ATP.