Porcine cytomegalovirus (PCMV) is one of the pathogens that should be eliminated from pigs intended for use as organ donors in xenotransplantation. For this purpose, reliable diagnostic test systems are needed. To provide a basis for this goal and to analyse the evolutionary relationships of PCMV within the herpesvirus family, the putative glycoprotein B (gB) gene of PCMV was identified by assuming gene colinearity and a relative conservation of nucleotide sequences in comparison with closely related herpesviruses. Using this approach the complete nucleotide sequence of the PCMV gB gene was determined. A protein of 860 amino acids was deduced and a putative cleavage site, conserved cysteine residues, as well as potential N-terminal glycosylation motifs were identified. In a comparison of PCMV gB with the corresponding region of other herpesviruses, the highest identities were found with human herpesviruses 6 and 7 (HHV-6 and 7; 43.4% and 42.6%, respectively). Also in phylogenetic analysis, the PCMV gB clustered with HHV-6 and HHV-7. Between the complete gB sequences of five different PCMV strains and isolates from the United Kingdom, Germany, Spain, Japan and Sweden, differences of 3.4% were found, indicating a considerable intra-species variation. The characterisation of the protein deduced from the identified gene provides further evidence that this is indeed the gB gene of PCMV and provides important taxonomical information regarding PCMV. The identification of the gB gene should facilitate the development of sensitive and robust diagnostic methods for the PCMV screening of pigs.