The invention relates to a recombinant DNA molecule coding a polypeptide having phytase stability after being expressed in a prokaryotic or eukaryotic host cell. Said recombinant DNA molecule comprises a DNA sequence selected from among a) DNA sequences coding a polypeptide that has phytase activity and is obtained by varying the mature wild-type E. coli phytase sequence, the variation being selected from among i) the mutation lysine→aspartic acid in position 74 (K74D), and/or ii) a combination of the mutations asparagine→arginine in position 139 (N139R) and aspartic acid→glutamic acid in position 142 (D142E), and/or iii) a combination of the mutations leucine→isoleucine in position 145 (L145I) and leucine→isoleucine in position 198 (L198I), and/or iv) a mutation valine→proline in position 200 (V200P), and/or v) an N-terminal or C-terminal or an N-terminal and C-terminal addition of a sequence section of the acidic phosphatase of Aspergillus niger var. awamori or the phytase of Aspergillus niger, b) DNA sequences that are 70 to 100 percent homologous to the sequences listed under a), c) DNA sequences which are related to the sequences listed under a) and b) because of the degeneration of the genetic code. When expressed in a suitable host cell, the recombinant DNA molecule has increased temperature stability as well as increased protease stability of the enzyme activity of the protein coded in said manner. Variations iii) and iv) are provided only in combination with a variation i), ii), and v). The invention also relates to the proteins coded thereby.