275 isolates of Ophiostoma novo-ulmi, sampled across the southern Great Lakes region of North America from Wisconsin to Ohio in 1996, were analysed for vegetative compatibility (vc) types. Over 60% of the sample was a clonal vc component, comprising only two vc types (the AMSG and EUSG); the remainder of the sample was highly heterogeneous for vc types. Vc diversity was highest in Indiana and Michigan; close to where NAN O. novo-ulmi probably first appeared in North America. Each vc clone exhibited a uniform RAPD haplotype and a lower frequency of the A compared with the B mating type. This is consistent with the clones being mainly asexually spread, but with a potential for inbreeding, an unusual feature in a ‘genetic clone’. The heterogeneous component, however, exhibited diverse RAPD haplotypes and a near 1[ratio ]1 ratio of A[ratio ]B mating types, indicating that the mainly novel vc types of this component arise via sexual recombination. In Europe, dominant vc clones have been quickly replaced by novel vc types. In the Great Lakes region, however, vc clones appear to have survived for over 50 years despite a high potential for emergence of new vc types via sexual recombination. No differences were found between the clonal and heterogeneous components with regard to growth rate and pathogenicity, two important fitness parameters. Two other factors, a low level of selection imposed by deleterious d-factor viruses and a density dependent effect associated with vegetative incompatibility, may have favoured prolonged survival of the clones in North America.