The envelope glycoprotein E2 of classical swine fever virus (CSFV) plays multiple roles in the viral life cycle, interaction with host and pathogenesis. We sequenced the E2 gene of 34 CSFV isolates from southeastern China for analysis of genetic diversity in this particular region. Phylogenetic analysis revealed that genotype 2.1b viruses became predominant in southeastern China with 33 isolates clustered in 2.1b and only 1 isolate belonged to 2.2. Pairwise comparisons demonstrated isolates in this study showed a homology of 78.4-82.6% to Chinese C-strain in the 190nt of the E2 fragment. The minimum similarity within the 2.1b isolates was 91.1%. Variability of the full length E2 amino acid sequences of 45 CSFV isolates representative of three main groups of CSFV including 19 from southeastern China during 2004--2007 and 26 from other parts of China and other countries was analyzed by the differences between non-synonymous (dN) and synonymous (dS) rates and the entropy values. Two variable and three conserved regions as well as some specific positions under positive selection were defined. Our results suggest that recent isolates from southeastern China have shifted away from historical CSFV isolates including the vaccine strains probably as a result of their adaptive abilities to the selection forces within the host.