The two subpopulations of the Dutch elm disease pathogen Ophiostoma novo-ulmi, previously known as the Eurasian (EAN) and North American (NAN) races, are redesignated as subspecies novo-ulmi and americana. In addition to their partial reproductive isolation, wide range of physiological and molecular differences and different geographic ranges, the two subspecies can be discriminated by their perithecial form and dimensions. These perithecial differences are described, using perithecia produced in multiple intra-subspecies crosses. Perithecia of subsp. novo-ulmi have an average neck length of ca 450 μm, base width of ca 103 μm and neck length:base width ratio ca 4.4. Perithecia of subsp. americana have an average neck length of ca 295 μm, base width ca 116 μm and neck length:base width ratio ca 2.6. The average shape and dimensions of subsp. americana perithecia is similar to that of O. ulmi. The average perithecial form of subsp. novo-ulmi, as well as of O. himal-ulmi, is rather distinctive. The current known geographical distribution of subspecies novo-ulmi and americana, based on > 6500 samples, is presented.