Abstract: Four H9N2 avian influenza viruses were isolated from sick chickens in Northern China．The sequences of the hemagglutinin genes of these isolates were determined by RT-PCR and sequencing．The homology of these nueleotide 8e—
quences and putative amino acid sequences were compared with H9N2 in GenBank．Phylogenetic tree based on different regions in nucleotide sequences ofHA gene cDNA was also analyzed．Although the isolating time of four viruses was different，they were highly homologous and all belong to the same sublineage of H9N2 Eurasian lineage，A／CK／HK／G9／97一like group．Their amino acid sequences that involved in HA cleavage sites，receptor binding sites and potential glycosylation sites were also similar to the above sublineage．However，two of the four isolated viruses，CK SD B1 98 and CK BJ L1 05
had a deletion of potential glyeosylation site because of a single nucleotide variation on HA gene．

Key words: avian influenza virus, H9N2 subtype, hemagglutinin gene, phylogenetic analysis