The envelope glycoprotein B (gB) coding sequences of two strains of human herpesvirus-6 (HHV6 GS and Z29) were determined by sequencing a 2.5-kb open reading frame adjacent to the DNA polymerase sequence. The deduced primary translation product is 830 amino acids in length and is 96% conserved between the two divergent strains with no localized hypervariability noted. It contains the expected signal and transmembrane sequence motifs as well as a putative site of protease cleavage. There was 39% amino acid identity with the gB of human cytomegalovirus (CMV, strain AD169). HHV6-CMV gB peptide homology was evident through the entire sequence, but was especially strong in the amino-terminal portion of CMV gp55, which contains linear and conformational epitopes recognized by CMV-neutralizing antibodies. All 10 cysteine residues of HHV6 gB match corresponding residues of CMV gB. Sequence data suggest strong structural similarity and possible immunologic cross-reactivity of gB from the two viruses.