A phylogenetic tree based on the VPI sequences of type O foot-and-mouth disease virus (FMDV) has been derived. Direct sequencing of PCR products has been used to obtain the VP1 gene sequences of new isolates. The tree exhibits four main lineages that largely correlate with the geographical origin of isolates. The analysis supports a close relationship between European O1 field isolates and vaccine strains, with the exception of O Thalheim Aus/81 and O Wuppertal Ger/82 which were probably of non-European origin. Analysis of nucleotide substitutions indicates that synonymous mutations play a major role in FMDV evolution.