Abstract: Objective To understand the circulation of influenza A (H3N2) virus and the antigen variation of the HAl domain of the virus in Ningbo in 2009. Methods The virus was isolated from the influenza like illness cases in sentinel hospitals every week during the year, the nucleotide sequencing of HA1 was conducted for H3N2 isolates and their amino acid sequences were deducted to conduct genetic characteristics analysis. Results The full length of HAl of 15 H3N2 strains was 987 bp, coding protein of 329 amino acids. Compared with the circulating strain in the autumn of 2008, 7 amino acids loci were different in the first half of 2009, and 10 amino acids loci were different between the circulating strain in the first half and that in the second half of 2009. The strains isolated in the first half and in the second half of 2009 belonged to different clades. Conclusion Firstly, compared with the strains isolated in 2008, there were several obvious variations in the HAl domain of strains isolated in the first half and in the second half of 2009; secondly, there were distinct differences between the strains isolated in the first half and in the second half of 2009, thirdly, the influenza A (H3N2) virus varied frequently during 2009 in Ningbo