A method for genetic grouping of classical swine fever viruses (CSFV) was developed based on the restriction fragment length polymorphism (RFLP) revealed by AvaII, BanII and PvuII digestion of RT-PCR amplified segments of the E2 gene. From inspection of the genetic sequences of Thai isolates and reference strains, the RFLP method was designed to be capable of differentiating all known genogroups and subgenogroups suggested by phylogenetic analysis of the CSFV E2 gene. The method was applied to 60 CSFV samples which included three genogroups and seven subgenogroups. Unlike previously described RFLP methods, the agarose gel patterns obtained from these samples were completely in agreement with the predicted RFLP patterns and enabled accurate genetic grouping of CSFV at the subgenogroup level. The simplicity of this method allows rapid CSFV genogrouping at diagnostic laboratories without sequencing facilities and provides a useful method for diagnosis as well as epidemiological investigation and control of classical swine fever outbreaks.