The frequency (frq) locus encodes a key component, a state variable, in a cellular oscillator generating circadian rhythmicity. Two transcripts have been mapped to this region, and data presented here are consistent with the existence of a third transcript. Analysis of cDNA clones and clock mutants from this region focuses attention on one transcript encoding a protein. FRQ, which is a central clock component: (i) mutations in all of the semidominant frq alleles are the result of single amino acid substitutions and map to the open reading frame (ORF) encoding FRQ; (ii) deletion of this ORF, or a frameshift mutation within it, results in a strain with a recessive clock phenotype characterized by the loss of rhythm stability and compensation. Single amino acid substitutions within, or disruption of, this single ORF are thus sufficient to drive major alterations in both period length and temperature compensation, two canonical characteristics of circadian systems. The 989-amino acid FRQ protein species the circadian function of frq in the assembly of the Neurospora biological clock.