The genetic diversity of classical swine fever virus (CSFV) was studied by RT-PCR amplification and sequencing of a 409 bp fragment of the NS5B polymerase region. A total of 106 viruses isolated from 20 countries over a period of 52 years (1945-1997) were included in the phylogenetic study. The results showed that the viruses could be divided into two main groups. Group 1 consisted of Asian and South American isolates from the 1980s, as well as of old European and American isolates. Group 2 consisted mostly of recent European viruses from the 1980s and 1990s, and was further divided into three subgroups largely according to geographic origin and/or year of isolation. Five 1997 CSFV isolates from Germany, Netherlands and Italy clustered together indicating a common origin for these outbreaks, but two other 1997 isolations in different regions of Germany are likely due to different epidemiological events. The results show that the NSSB region of the genome gives a good resolution for phylogenetic studies of CSFV. Molecular epidemiology based on nucleotide sequence diversity is a useful tool for tracing virus spread and for developing disease control strategies.