ABSTRACT Two isolates of the barley net blotch pathogen (Pyrenophora teres f. teres), one possessing high virulence (0-1) and the other possessing low virulence (15A) on the barley cultivar Harbin, were crossed and the progeny of the mating were isolated. Conidia from cultures of the parent and progeny isolates were used as inoculum to determine the inheritance of virulence in the pathogen. Of the 82 progeny tested, 42 exhibited high virulence and 40 exhibited low virulence on 'Harbin' barley. The data support a model in which a single, major gene controls virulence in P. teres f. teres on this barley cultivar (1:1 ratio; chi(2) = 0.05, P = 0.83). Preparations of DNA were made from parental and progeny isolates, and the DNA was subjected to the random amplified polymorphic DNA (RAPD) technique in a search for molecular genetic markers associated with the virulence phenotype. Five RAPD markers were obtained that were associated in coupling with low virulence. The data indicate that the RAPD technique can be used to tag genetic determinants for virulence in P. teres f. teres.