The developmental behavior of cloned histone genes of Psammechinus miliaris was studied by injection into eggs of two related sea urchin species followed by fertilization. All five early histone genes were faithfully expressed in early blastula embryos as shown by SP6 RNA mapping. A 5-10 times lower expression rate was estimated for the injected early H2A gene from its competition strength with the endogenous gene. Transcripts of this early H2A gene accumulated during the cleavage stages and decayed in late embryos in parallel with the endogenous early H2A mRNA. However, an introduced late H2B gene was incorrectly regulated, since its mRNA level did not increase from the blastula to the gastrula stage. The sperm H2B-1 gene, normally inactive in development, was 80 times less well expressed than the early H2A gene in transformed blastulae. A fusion gene with the early H2A promoter linked to the structural sperm H2B gene was, however, efficiently transcribed suggesting that all essential information for an early expression pattern is contained within the 5' region of the early H2A gene.