The nucleotide sequences of two chicken histone genes encoding replacement variant H3.3 polypeptides are described. Unlike the replication variant genes of chickens (and almost all other organisms), these genes contain intervening sequences; introns are present in both genes in the 5' noncoding and coding sequences. Furthermore, the replacement variant histone mRNAs are post-transcriptionally polyadenylated. The locations, but not the sizes, of the two introns within the coding segments of the two genes have been exactly conserved, whereas the intron positions in their respective 5' flanking regions differ. Although both H3.3 genes predict the identical histone polypeptide sequence, they are as different from one another as each of them is from a more common replication variant H3.2 gene in silent base substitutions within the coding sequences. Thus, the H3.3 polypeptide sequence has been precisely maintained over a great evolutionary period, suggesting that this class of histones performs a strongly selected biological function. Although replacement variant histones can account for more than 50% of the total H3 protein in the nuclei of specific chicken tissues, the steady-state level of H3.3 mRNA is nearly the same (and is quite low) in all tissues and ages of animals examined. These properties suggest novel mechanisms for the control of the basal histone biosynthesis which takes place outside of the S phase of the cell cycle.