Expression of the phoA gene cloned in a multicopy plasmid in Escherichia coli E15 (Delta phoA) results in overproduction of the alkaline phosphatase. Secretion occurs into the periplasm and the culture medium, and accumulation of a precursor (prePhoA) occurs as insoluble aggregates. All these forms of the enzyme were purified and their properties were studied. Extracellular and periplasmic enzymes differ in V-max and K-m values but have similar activation energies (E(a)). Mature alkaline phosphatase and intermediate forms can be isolated from one E. coli culture.