A gene pair coding for histones H3 and H4 expressed during late embryonic development has been cloned from the S. purpuratus genome. The organization of these genes is similar to the divergently transcribed H3-H4 gene pairs of Lytechinus pictus. Whole genome Southern analysis indicates that most late H3 and H4 genes are organized as pairs in both sea urchin genomes. The nucleotide sequences of late and early S. purpuratus H3 and H4 genes differ in the coding regions by 17.0% and 15.7%, respectively. Although there is little match between early and late genes outside the transcription unit, there are short stretches of homology in the spacers 5' to the S. purpuratus early and late H3 genes and the early and late H4 genes. Sequence comparison of the late H3-H4 S. purpuratus pair with two late H3-H4 pairs from L. pictus reveals additional striking homologies in the intergenic spacer. At least four late H3 and two H4 RNAs are distinguished by hybridization to Northern electroblots with probes derived from the cloned gene pair. Although most of these RNAs appeared to accumulate coordinately, with the most dramatic increase occurring between 13 and 17 hours of development, some differences in timing of appearance of different H3 RNA species are observed.