The ciliate Tetrahymena thermophila has been successfully used as a model system in molecular and cell biology for decades. Fundamental discoveries such as ribozymes, telomeric repeats, telomerases or the function of scan RNAs were first studied in this eukaryotic microorganism. In addition, cells grow fast to high cell densities in inexpensive media and simple bioreactor infrastructure and several foreign proteins have been expressed, suggesting that T. thermophilahas the potential to become an excellent expression host.