1. Clones failing to grow on proline were isolated after ethylmethane-sulfonate treatment of a glutamate auxotroph of Saccharomyces, originally capable of utilizing proline as principal nitrogen source. 2. Most of the non-utilizers of proline were cytochromeless. One, however, possessed normal cytochrome levels but lacked Δ 1-pyrroline-5-carboxylate dehydrogenase activity. This is the first isolate of this lesion in Saccharomyces.