The present application relates to mutated RNA polymerases from bacteriophages that have increased stability, for example under high temperature conditions. Preferred mutated RNA polymerases according to the invention are mutant RNA polymerases from T7 or SP3 bacteriophages. An especially preferred embodiment of the present invention is a T7 RNA polymerase with a serine to proline amino acid change in the protein at position 633 of the amino acid sequence.