The gene encoding the envelope glycoprotein of Omsk haemorrhagic fever (OHF) virus was cloned and sequenced. A freeze-dried preparation of infected suckling mouse brain suspension was used as the source material for viral RNA. The derived cDNA was amplified using the polymerase chain reaction and the cloned DNA sequenced by dideoxynucleotide sequencing. Alignment of the OHF virus sequence with those of other known tick-borne flaviviruses showed that they shared N-glycosylation sites, cysteine residues, the fusion peptide and a hexapeptide (EHLPTA) that identifies tick-borne flaviviruses. OHF virus was distinguishable from the other viruses but shared closest amino acid identity (93.0%) with the tick-borne encephalitis viruses. A sequence of three amino acids (AQN; amino acids 282 to 234), which was previously shown to be specific for the tick-borne encephalitis viruses, was altered to MVG in OHF virus. This is predicted to have a higher hydrophobicity than the AQN sequence and may therefore have significant implications for the phenotypic characteristics of OHF virus. The results demonstrate close phylogenetic relationships between these viruses but also show their distinct evolutionary development. Sequence changes within the envelope glycoprotein of OHF virus have been identified that may be responsible for the distinct tropism of this flavivirus. These results support and enlarge upon previous data obtained from serological analysis.