The phosphate-binding protein (PhoS) is a periplasmic protein which is part of the high-affinity phosphate transport system of Escherichia coli. Hyperproduction of PhoS in strains carrying a multicopy plasmid containing phoS led to partial secretion of the protein. By 6 h after transfer to phosphate-limiting medium, about 13% of the total newly synthesized PhoS was secreted to the medium. Kinetic studies demonstrated that this secretion consists of newly synthesized PhoS. This secretion occurs in PhoS-hyperproducer strains but not in a PhoS-overproducer strain. Another type of secretion concerning periplasmic PhoS was observed in both PhoS-hyperproducer and PhoS-overproducer strains. This mode of secretion depended upon the addition of phosphate to cells previously grown in phosphate-limiting medium.