Abstract
Strains E, S and W of Tetrahymena pyriformis were examined for their ability to carry out the reactions of the Krebs‐Henseleit urea cycle, using growth and enzyme studies. None of the strains was able to grow on either citruliine or ornithine in place of arginine, and proline was as active as citrulline or ornithine in sparing arginine. So little citrulline or arginine was synthesized by cell‐free preparations as to be of no significance in the growth or nitrogen metabolism of the ciliates. Slight arginase activity could be detected in homogenates, but no urea was found in cultures. No urease activity could be detected using urea‐C14.