The set of histone genes that are active late in embryogenesis of the sea urchin (Strongylocentrotus purpuratus) are present in 5-12 copies per genome and, unlike the clustered, tandemly arrayed early histone genes, are dispersed and irregularly arranged. Late H2B gene expression is activated by events accompanying fertilization and its mRNAs a, first detectable by as early as 6 h of development (16 cells) and increase only slightly in amount during the period of rapid cleavage between 6 and 14 h. However, during the short interval between 14 and 16 h, while the amount of early histone mRNA is declining, there is a greater than 15-fold burst in the rate of late H2B mRNA accumulation.