One-hundred-and-thirteen isolates of Salmonella serotype Thompson from diverse sources in seven countries were characterized by PvuII ribotyping and IS200 fingerprinting. Ten PvuII ribotypes were observed. The predominant PvuII ribotype 1 represented a major clone of world-wide distribution but was not found in Australia; PvuII ribotypes 2 and 3 represented minor clones. HincII ribotyping discriminated subtypes within PvuII ribotype 1: HincII ribotype 1 was distributed widely but HincII ribotype 2 was found mainly in Scottish isolates. None of 101 isolates of PvuII ribotypes 1-3 contained copies of IS200. All 12 isolates of PvuII ribotypes 4-10 were from Australia and 7 of them contained copies of IS200 of 5 different profiles. These results suggest the existence of at least two lineages of Salmonella Thompson with a different geographical distribution. The finding that most isolates from man and poultry in Scotland belonged to the same ribotype (PvuII 1/HincII 2) and were IS200-negative suggests that poultry is an important source of human infection in Scotland.