Ornithine decarboxylase (ODC) (EC 4.1.1.17) is an early enzyme of polyamine synthesis, and its activity rises quickly at the onset of growth and differentiation in most eucaryotes. Some have speculated that the enzyme protein may have a role in the synthesis of rRNA in addition to its role in catalyzing the decarboxylation of ornithine (G. D. Kuehn and V. J. Atmar, Fed. Proc. 41:3078-3083, 1982; D. H. Russell, Proc. Natl. Acad. Sci. USA 80:1318-1321, 1983). To test this possibility, we sought mutational evidence for the indispensability of the ODC protein for normal growth of Neurospora crassa. We found three new, ODC-deficient mutants that lacked ODC protein. Among these and by reversion analysis of an earlier set of mutants, we found that two ODC-deficient mutants carried nonsense mutations in the ODC structural gene, spe-1. Allele LV10 imparted a complete deficiency for enzyme activity (less than 0.006% of normal) and had no detectable ODC antigen. Allele PE4 imparted a weak activity to cells (0.1% of derepressed spe+ cultures) and encoded a lower-molecular-weight ODC subunit (Mr = 43,000) in comparison to that of the wild-type strain (Mr = 53,000). Strains carrying either mutation, like other spe-1 mutants, grew at a normal rate in exponential culture if the medium was supplemented with spermidine, the main end product of the polyamine pathway in N. crassa. Unless an antigenically silent, N-terminal fragment with an indispensable role persists in the LV10-bearing mutant, we conclude that the ODC protein has no role in the vegetative growth of this organism other than the synthesis of polyamines. The data extend earlier evidence that spe-1 is the structural gene for ODC in N. crassa. The activity found in mutants bearing allele PE4 suggests that the amino acids nearest the carboxy terminus do not contribute to the active site of the enzyme.