The genetic relatedness of 7 Korean type O field strains of foot-and-mouth disease virus (FMDV) in clinical specimens collected from 5 different geographic locations in 2000 was investigated. The sequence of 162 nucleotides (nt 478-639) at the 3' end of the 1D (VP1) genes was determined from amplified cDNA fragments, and subjected to the analysis for the sequence identity/divergence and phylogenetic relationship. The overall nucleotide sequence divergence among the 7 field strains was 0 to 3.8%, suggesting that they are closely related to each other. Phylogenetic analysis with the known Middle East-South Asia (ME-SA) topotype strains showed that the 7 Korean field strains formed two distinct clusters within the same lineage of the ME-SA topotype strains. Cluster 1 consisted of the strains of the primary foci of infection (Paju and Hongseong), and closely related to the strains prevailed in the Far East. Cluster 2 comprised those of subsequently affected regions (Boryeong, Yongin, and Chungju), and was further diverged from the Cluster 1. The result of phylogenetic analysis indicated that the Korean strains may have evolved from a common ancestor of the Pan Asia strains, and that at least 2 phylogenetically clustered variants within the same lineage were prevalent during the epidemic. The potential origin and sources of the virus introduction to Korea were discussed.