The structures of European and North American populations of Ophiostoma ulmi , responsible for the first pandemic of Dutch elm disease, were compared by assessing the diversity of vegetative compatibility (vc) types and sexual mating types. All six European samples examined were highly diverse for vc-types whether the samples were continental, regional, or very local in origin. Overall, the A and B sexual mating types occurred in roughly equal frequency in Europe. In contrast, a single vc-type predominated in both a part-continental and a regional sample from North America ( ca 74 and 94% respectively). This was termed the O. ulmi North American vc supergroup. In this vc-type the A sexual compatibility type was dominant ( ca 65 and 82% respectively). The small remainder of North American isolates mainly comprised diverse vc-types, most of which were B-types. The North American vc supergroup was also the largest single vc group found locally in Europe. Circumstantial evidence suggests that the O. ulmi vc supergroup may have been present in North America since at least the 1940s. Its existence may provide a clue to the time and place of the first appearance on the North American continent of O. novoulmi , responsible for the current second pandemic of the disease. Possible reasons for the differences between present day North American and European O. ulmi populations are discussed. It is suggested that the O. ulmi North American vc supergroup may once have been the dominant vc-type in Europe as well as in North America, and that the European O. ulmi population may have become diversified in vc-types when the first epidemic in Europe declined during the 1940s.