The aim of this study was to understand the evolutionary relationships amongst Old World primate lymphocryptoviruses (LCVs). To this end, the chimpanzee LCV, gorilla LCV, orang-utan LCV and baboon LCV homologues of the Epstein-Barr virus (EBV) BALF4 gene, encoding glycoprotein B (gB) were sequenced. The gB proteins from Old World primate LCVs were between 85-99% identical and each contained ten conserved cysteine residues at equivalent positions. The chimpanzee and orang-utan LCV gB proteins were found to be 100% similar and 99% identical suggesting they were in fact from isolates of the same virus (either chimpanzee or orang-utan LCV) that had resulted from a contamination event. For the purposes of this study they were therefore regarded as the same virus, and the chimpanzee and orang-utan LCV sequences were referred to as chimp/orang LCV gB. Protein alignments of derived amino acid sequences suggested most variation in these sequences occurred in predicted loop regions. Phylogenetic and comparative analyses of the sequences obtained in this study, plus the previously published EBV and rhesus LCV sequences were undertaken. Additionally, the Old World virus sequences were rooted using the New World primate LCV, Callitrichine herpesvirus 3 (CalHV-3), gB sequence as an outgroup. The results obtained suggested the existence of one clade comprising gorilla LCV gB and chimp/orang LCV gB (gorilla LCV clade), whose hosts belong to the family Hominidae, and tentatively suggested the existence of another clade comprising rhesus LCV gB, baboon LCV gB and EBV gB (rhesus LCV clade), whose hosts, aside from EBV, belong to the family Cercopithecoidae. These data suggested that, aside from EBV, these viruses had co-evolved with their hosts, and that the root of the tree lay between these two clades. Analysis of the amount of evolution that has occurred within each virus lineage indicated that a significantly faster rate of evolution had occurred within each clade rather than between each clade, suggesting that the rate of gB evolution had accelerated following co-speciation. Rates of evolution were calculated between the rhesus LCV/baboon LCV and gorilla LCV/baboon LCV BALF4 sequence pairs, and compared to mean rates of change calculated using the gorilla LCV and chimp/orang BALF4 sequences. All calculations assumed these viruses had co-speciated with their hosts. The rates obtained for the gorilla LCV BALF4-chimp/orang BALF4 distances were reasonably similar to those obtained using the rhesus LCV/baboon LCV divergence, corroborating earlier evidence that the rhesus LCV and baboon LCV BALF4 sequences had co-speciated with their hosts. The position of EBV gB on the tree was more difficult to explain. The sharing of an identical four amino acid deletion in the EBV and rhesus LCV proteins suggested two possible scenarios. Either EBV BALF4 had been acquired from the rhesus LCV genome by recombination or there had been an inter-species transfer of rhesus LCV to humans. Phylogenetic analyses of other available genes showed that rhesus LCV and EBV did not group together. This result was compatible with acquisition of EBV BALF 4 from the rhesus LCV genome by recombination.