The complete nucleotide sequence and genomic organization of human papillomavirus type 18, associated with cervical cancer, has been established. A detailed comparative analysis was undertaken leading to the identification of a number of features specific for genital papillomaviruses and the construction of a phylogenetic tree. Genital papillomaviruses differ from other human and animal papillomaviruses as they possess a longer E1 open reading frame (ORF) and have a characteristic control region. Phylogenetically, HPV 18 is located between the benign genital viruses, HPV 6 and HPV 11, and the malignant isolates, HPV 16 and HPV 33, and may represent an evolutionary intermediate among oncogenic papillomaviruses. Viral gene products known to be involved in cellular transformation are those of ORFs E5, E6 and E7. Significant sequence variation was found between the E6 to E7 regions of different integrated forms of HPV 18. On re-examination of the E6 primary structures we noticed that the gene has evolved by successive duplications of a unit encoding 33 amino acids, which include a Cys-X-X-Cys motif. Furthermore, the E7 gene product has apparently evolved in the same manner and is related to E6. Both gene products bear a striking resemblance to the transcriptional factor IIIA of Xenopus laevis, the prototype of a new class of nucleic acid binding proteins.