The nucleotide sequence of the glycoprotein G (gG) homologue of asinine herpesvirus 3 (AHV3), a respiratory alphaherpesvirus of donkeys, was determined. The AHV3 gG gene consists of 1233 base pairs (bp) and codes for a predicted protein of 411 amino acids. This is identical in size to the equine herpesvirus 1 (EHV1) gG gene and 6 amino acids longer than the equine herpesvirus 4 (EHV4) gG gene. The predicted amino acid sequence of AHV3 gG has characteristics of a class 1 membrane protein. The amino acid sequence of AHV3 gG shows 92% and 60% identity to EHV1 gG and EHV4 gG respectively. Two regions within the gG amino acid sequences of EHV1 and EHV4 were previously defined, an N-terminal constant region and an immunodominant highly variable region located toward the C-terminus. In the corresponding constant region of AHV3 gG there was 96% and 75% amino acid identity with EHV1 and EHV4 gGs respectively. In the variable region, there was 73% and 24% identity respectively. Phylogenetic analyses using the gG nucleotide sequences indicated that AHV3 is much closer in evolutionary distance to EHV1 than either virus is to EHV4. These findings provide additional support for the view that AHV3, or another closely related virus, may be the progenitor of EHV1 and has adapted to horses in relatively recent times.