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An analysis of the SalI restriction pattern of African swine fever virus DNA showed that the SalI recognition sites did not change after more than 100 virus passages in porcine macrophages. The virus strain BA71V, obtained from the virus isolate BA71 by adaptation to grow in VERO cells, differed from the nonadapted virus in two deletions with a length of 2.5 and 7 kb located close to the DNA ends. A restriction analysis of several virus clones obtained from a naturally infected pig revealed length heterogeneity in both variable regions. A comparison of SalI restriction maps from 23 African swine fever virus field isolates (8 African, 11 European, and 4 American) has shown that the virus genome consists of a central region with a constant length of about 125 kb and two variable regions located close to the DNA ends with a length of 38-47 kb for the left DNA end, and 13-16 kb for the right DNA end. The total length of ASF virus DNA varied between 178 (BA71) and 189 (MOZ64) kb. The 23 African swine fever virus isolates were classified into five groups, according to the arrangement of the SalI sites in the central region. Four groups contained only African isolates, whereas all the European and American isolates belonged to the same group. This distribution of isolates suggests that all non-African virus field isolates have a common origin.