The arrangement of the polychaete annelid Platynereis dumerilii core histone gene cluster and nucleotide sequence has been reported (D. Sellos, S. A. Krawetz and G. H. Dixon, 1990). The H2B and H3 mRNAs are transcribed from one DNA strand, while the H2A and H4 histone mRNAs are transcribed from the other. The H1 gene is not contained as a member of this cluster. Computer assisted sequence analysis of this region was undertaken to define the organization and representation of the various sequence motifs embedded within this region. The analysis revealed that two large regions on opposite strands of the cluster were similar to one another. This organization is reminiscent of an ancient gene duplication event from which the various members independently evolved.