Mutants with a feedback resistant N-acetylglutamate synthase have been isolated from a proA/B, argD, argR strain by screening for proline excretion on minimal medium with arginine. The feedback resistant character of three mutants was transduced into an argA (N-acetylglutamate synthase negative) strain. It was cotransducible with argA at a frequency of greater than 99%. N-acetylglutamate synthase extracted from the three mutants was approximately one hundred times less sensitive to L-arginine than the enzyme from the feedback sensitive parent strain.