phoU mutants of bacteria are potentially useful for the removal of inorganic phosphate (P-i) from sewage because they can accumulate a large amounts of polyphosphate (polyP). However, the growth of phoU mutants is severely defective and is easily outgrown by revertant(s) that have lost the ability to accumulate polyP during growth in a nutrient-rich medium. We found that a pseudo-revertant, designated LAP[+], that appeared in a culture of an Escherichia coli phoU mutant that could accumulate polyP even after ten serial passages. Reduction in the expression of the P-i-specific transporter Pst in LAP[+] may contribute to relieving stresses such as excess P-i incorporation that could stimulate reversions. The discovery of a LAP[+] provides a clue to generate phoU mutants that accumulate polyP in a stable manner.