In narrow-leafed lupin (Lupinus angustifolius), segregation for the necrotic (systemic hypersensitive) response to infection with a necrotic strain of Bean yellow mosaic virus (BYMV-N) was studied in progeny plants from six crosses. The parents were two cultivars that always developed necrosis when infected (Danja and Merrit) and two genotypes that always responded without necrosis (90L423-07-13 and P26697). In the four possible combinations of crosses between the different necrotic and non-necrotically reacting genotypes, segregation for the necrotic response in F 2 progeny plants always fitted a 3:1 ratio (necrotic: non-necrotic). All F 2 progeny plants from the cross between the two noncultivar genotypes became infected without necrosis while 99% of the F 2 from the cross between the two cultivars developed necrosis. These results indicate that the systemic necrotic response to infection with BYMV-N is probably controlled by a single dominant hypersensitivity gene for which we propose the name Nbm-1. However, its expression seemed influenced by independently segregating modifier genes in the genetic background since necrosis developed at widely different rates within affected F 2 progeny plants resulting in staggered killing.