A single nuclear gene controls nonsporulation in a novel isolate of the Dutch elm disease fungusOphiostoma ulmi(Buism.) Nannf. This has been clearly demonstrated through segregation of the nonsporulating phenotype-in meiotic products recovered from crosses between a mutant nonsporulating isolate (WRB2-1) and wild-type sporulating isolates, between F1progeny and their parents, and between F1progeny. All crosses between nonsporulating and sporulating isolates yielded a 1:1 ratio for these two phenotypes in the meiotic products, whereas all crossings between isolates of the same phenotype produced meiotic products of that phenotype. The genetic stability of the nonsporulating phenotype was clearly shown when no disease symptoms were observed following artificial inoculation of the nonsporulating progeny into white elm,Ulmus americanaL. Exposure to trifluoperazine, a calmodulin inhibitor, did not shift the nonsporulating isolates to the yeast phase, which supports our findings that nonsporulation is under genetic control rather than metabolic control.Key words: nonsporulation,Ophiostoma ulmi, mutant, single nuclear gene, meiotic products.