The present invention relates to polypeptides having phytase activity. These polypeptides have an amino acid sequence which has at least 70% identity to either of three phytases derived from the bacterium Buttiauxella, and which comprises at least one of the following amino acids at the position indicated: 119N, 120L, and/or 121E. These phytases have an improved specific activity. Additional specific amino acid substitutions are also disclosed which characterize and distinguish additional phytases of the invention having improved properties such as temperature and/or pH stability, pH activity profile, temperature activity profile, substrate profile, improved performance in animal feed in vitro or in vivo. The invention also relates to isolated polynucleotides encoding the polypeptides, nucleic acid constructs, vectors, and host cells comprising the polynucleotides as well as methods for producing and using the polypeptides.