ABSTRACT: The region of the viral haemorrhagic septicaemia virus glycoprotein gene coding for amino acids 142 to 357 was sequenced and compared among 6 isolates of the virus from rainbow trout in Denmark. Isolates were selected that were strongly neutralized by polyclonal and monoclonal antisera, not neutralized by antisera, or that represented more than 500 serial passages in cell cultures. The overall diversity within this region of the glycoprotein was 5.4% at the nucleotide level and 6.9% at the amino acid level. Most of the variation was in the portion of the protein from amino acids 210 to 290 where substitutions were found in 13 of the 80 amino acids (16%). In contrast, the central portion of the glycoprotein of the F1 reference strain of the virus was remarkably stable during 510 passes in cell culture, accumulating only a single amino acid substitution. Differences between the neutralizable and non-neutralizable isolates were found in the regions from amino acids 257 to 259 and from 283 to 288.