Molecular hybridization of nuclear DNAs has been employed to study the evolution of nonrepetitive DNA sequences in four species of sea urchin. The data indicate that the extent of homology between the nonrepetitive DNA sequences of S. purpuratus, S. droebachiensis, S. franciscanus, and L. pictus confirms the phylogenetic relationship established through palaeontological evidence. The average rate of divergence of nonrepetitive DNA sequences was found to be approximately 0.22% per million years. In addition, a small fraction (approximately 14%) of the nonrepetitive DNA sequences is highly conserved between S. purpuratus and L. pictus after 120-200 million years divergence. This study may provide a basis for the use of interspecific hybrid embryos of these organisms to investigate the evolution and importance of certain DNA sequences in early developmental processes leading to cell differentiation.