Laboratory strains of Ophiostoma ulmi carrying nuclear mutations induced by exposure to N-methyl-N′-nitro-N-nitrosoguanidine were crossed, and the segregation of genetic markers was analyzed in random ascospore progeny. Investigation of 13 auxotrophic mutations and 1 benomyl-resistant mutation provided evidence for at least three linkage groups in O. ulmi. Five loci, identified by mutant alleles ade1-1, BENIR-1, cyi1-1, lys3-1, and nic1-1, were assigned to linkage group I, whereas markers ade2-1 and lys2-2 were mapped on linkage group II. An additional locus, Met1, was assigned to a third linkage group since mutant alleles at this locus segregated independently from markers on group I or II. The Ben1R locus, controlling resistance to benomyl, segregated independently from the mating type locus and thus appeared to differ from the Tol locus described by other workers. Key words: Ophiostoma ulmi, Dutch elm disease, mutations, linkage analysis.