Cloned repetitive DNA sequences were used to determine the number of homologous RNA transcripts in the eggs of two sea urchin species, Strongylocentrotus purpuratus and S. franciscanus. The eggs of these species contain different amounts of RNA, and their genomes contain different numbers of copies of the cloned repeats. The specific pattern of repetitive sequence representation in the two egg RNA's is nonetheless quantitatively similar. The evolutionary conservation of this pattern suggests the functional importance of repeat sequence expression.