Cultures of Erysiphe graminis f. sp. hordei virulent in pathogenicity on Hordeum spontaneum and on barley varieties derived from H. spontaneum were isolated at the Plant Breeding Institute, Cambridge, in ig62 and subsequently used for genetical studies at Beltsville, Maryland, U.S.A. Reactions of the progeny of crosses between the variety Maris Concord (a derivative of H. spontaneum) and the varieties Kwan and Psaknon to infection with three cultures of E. graminis f. sp. hordei (63−12, 63−14 and CR3) suggested that a gene at the Mla locus on chromosome 5 of barley conditioned the reaction of the variety Maris Concord to these cultures. The same gene, or closely linked genes, conditioned the susceptible reaction to culture 63–14 of Maris Badger, Maris Concord and selection HB 279/5/1/2 derived from crosses with H. spontaneum and of other derivatives of H. spontaneum. Since the Cambridge varieties and selections used in these investigations, and H. spontaneum used in other studies have been found to have a gene conditioning their resistance at the Mla locus, and a complementary gene 'in the pathogen conditioned the pathogenicity on these hosts, the gene conditioning the resistance of Maris Concord (and the varieties Maris Badger and HB 279/5/1/2) and the gene JMlsn in H. spontaneum nigrum are assumed to be the same. The gene in the variety Maris Concord was shown to be different from the genes Mla, Mla2, Mla3, Mla4 and Mla5 since the varieties containing these genes were resistant to culture 63–14, whereas Maris Concord was susceptible. The gene conditioning the resistant reaction of Maris Concord (and Maris Badger and HB 279/5/1/2) and the gene JMlsn in Hordeum spontaneum nigrum have been designated Mla6.