Corresponding genomic regions of isolates of yellow head virus (YHV) from Thailand and gill-associated virus (GAV) from Australia were compared by RT-PCR and sequence analysis. PCR primers designed from sequences in the GAV ORFlb polyprotein gene amplified the corresponding 577 nucleotide region of the YHV genome. Comparison of the amplified region indicated 85.1 % nucleotide and 95.8% amino acid sequence identity. YHV PCR primers designed to amplify a 135 nucleotide product previously

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... as a YHV &agnostic probe failed to amplify the correspondmg product from GAV RNA. However, the cognate GAV sequence for this and another recently reported YHV sequence were located in an upstream region of the ORFlb gene. A comparison of these sequences indicated identities of 83.0 and 80.9% at the nucleotide level and 86.7 and 86.5% at the amino acid level, respectively. The data indicate that GAV and YHV are closely related but distinct viruses for whlch differential diagnostic probes can be applied.