A new strain of influenza A virus subtype H1N1 first identified in April 2009 has caused global pandemics . The first confirmed case in Shanghai was in May 2009. In order to study the characteristics of the 2009 influenza A virus subtype H1N1 in Shanghai , the virus stains A/ Shanghai/ 37T/ 2009 and A/ Shanghai/ 71T/ 2009, which were identified by real-ti me reverse transcriptase-polymerase chain reaction ( RT-PCR) , were isolated fromtwo i mported cases . Then , the characteristics of viral structure , drug  esistance , gene andsubtyping of the two viruses were studied by electron microscopy , i mmunofluorescence technique , genome sequencing , and bioinf ormation  oftware analysis . Virus particles from Madin- Darby canine kidney ( MDCK) cell culture showed that it contained numerous enveloped and highly pleomorphic orthomyxovirus  articles with a diameter ranging from60 nmto 80 nmand surrounded by a fringe of welldefined surface projection . The genome nucleotide sequence and amino acid  equence were then compared . The results showed that the two virus strains and the reference strain A/ Calif ornia/ 04/ 2009( H1N1) were highly homologous . Amantadine resistance occurred at residue 31 ( Asn) in M2 protein, while in the neuraminidase protein domain, it was sensitive to oseltamivir . Phylogenetic analysis showedthat thetwo strains were highly genetically si milar to those previously reported on the 2009 influenza A virus subtype H1N1 strains , suggesting the same origins f or the two Shanghai strains and 2009influenza A virus subtype H1N1 strains .