An isolated, synthetic or recombinant nucleic acid comprising (a) a nucleic acid sequence encoding a polypeptide having a phytase activity and having at least 95%, 96%, 97%, 98% or 99% or more sequence identity to SEQ ID NO:1, and having a nucleotide modification, wherein the nucleotides at positions 535 to 537 are CGT, CGC, CGA, CGG, AGA or AGG, wherein the polypeptide has improved thermostability over the phytase of SEQ ID NO:2; (b) a nucleic acid encoding a polypeptide having a phytase activity and having at least 95%, 96%, 97%, 98% or 99% or more sequence identity to SEQ ID NO:2, and having an amino acid modification wherein the amino acid glycine acid at amino acid position 179 of SEQ ID NO: 2 is changed to an arginine, wherein the polypeptide has improved thermostability over the phytase of SEQ ID NO:2; (c) the nucleic acid sequence of (a) or (b) encoding a polypeptide having a phytase activity but lacking: a signal sequence or proprotein sequence, or a homologous promoter sequence; (d) the nucleic acid of any of (a) to (c) encoding a polypeptide having a phytase activity and further comprising a heterologous amino acid sequence, or the nucleic acid of any of (a) to (c) further comprising a heterologous nucleotide sequence; (e) the nucleic acid of (d), wherein the heterologous amino acid sequence comprises, or consists of a sequence encoding a heterologous (leader) signal sequence, or a tag or an epitope, or the heterologous nucleotide sequence comprises a heterologous promoter or encodes a targeting sequence; or (f) a nucleic acid sequence completely complementary to the nucleic acid sequence of any of (a) to (e). (a) a nucleic acid sequence encoding a polypeptide having a phytase activity and having at least 95%, 96%, 97%, 98% or 99% or more sequence identity to SEQ ID NO:1, and having a nucleotide modification, wherein the nucleotides at positions 535 to 537 are CGT, CGC, CGA, CGG, AGA or AGG, wherein the polypeptide has improved thermostability over the phytase of SEQ ID NO:2; (b) a nucleic acid encoding a polypeptide having a phytase activity and having at least 95%, 96%, 97%, 98% or 99% or more sequence identity to SEQ ID NO:2, and having an amino acid modification wherein the amino acid glycine acid at amino acid position 179 of SEQ ID NO: 2 is changed to an arginine, wherein the polypeptide has improved thermostability over the phytase of SEQ ID NO:2; (c) the nucleic acid sequence of (a) or (b) encoding a polypeptide having a phytase activity but lacking: a signal sequence or proprotein sequence, or a homologous promoter sequence; (d) the nucleic acid of any of (a) to (c) encoding a polypeptide having a phytase activity and further comprising a heterologous amino acid sequence, or the nucleic acid of any of (a) to (c) further comprising a heterologous nucleotide sequence; (e) the nucleic acid of (d), wherein the heterologous amino acid sequence comprises, or consists of a sequence encoding a heterologous (leader) signal sequence, or a tag or an epitope, or the heterologous nucleotide sequence comprises a heterologous promoter or encodes a targeting sequence; or (f) a nucleic acid sequence completely complementary to the nucleic acid sequence of any of (a) to (e).