Two auxotrophs of Neurospora crassa have been isolated that give a positive growth response to putrescine, spermidine or spermine. One of the mutants is deficient in ornithine decarboxylase activity and has been designated put-I. Both mutants map on linkage group VR, fail to complement and are infertile when crossed to one another, indicating that they are probably alleles. A putrescine auxotroph is incapable of suppressing a pro-4 mutant. The isolation of the mutants confirms that putrescine is an essential factor the normal growth of the organism, and is synthesized via a single pathway in Neurospora.