The genetics of resistance to Phomopsis stem blight caused by Diaporthe toxica Will., Highet, Gams & Sivasith. in narrow-leafed lupin ( Lupinus angustifolius L.) was studied in crosses between resistant cv. Merrit, very resistant breeding line 75A:258 and susceptible cv. Unicrop. A non-destructive glasshouse infection test was developed to assess resistance in the F-1, F-2, selected F-2-derived F-3 (F-2:3) families, and in selfed parent plants. The F-1 of Unicrop x 75A:258 (and reciprocal cross) was very resistant, and the F-2 segregated in a ratio of 3:1 (resistant: susceptible), which suggested the presence of a single dominant allele for resistance in 75A:258. In Merrit x Unicrop (and reciprocal), the F-1 was moderately resistant, and the F-2 segregated in a ratio of 3:1 (resistant: susceptible). Thus Merrit appeared to carry an incompletely dominant resistance allele for resistance. The F-1 of Merrit x 75A:258 (and reciprocal) was very resistant and the F-2 segregated in a ratio of 15: 1 (resistant: susceptible), which supported the existence of independently segregating resistance alleles for resistance in 75A:258 and Merrit. Alleles at loci for early flowering (Ku) and speckled seeds (for which we propose the symbol Spk) segregated normally and independently of the resistance alleles. Resistant F-2 plants gave rise to uniformly resistant or segregating F-2:3 families, whereas susceptible F-2 plants gave rise only to susceptible F-2:3 families. However, the variation in resistance in the F-2 and some F-2:3 families of crosses involving 75A:258, from moderately to extremely resistant, was greater than that expected by chance or environmental variation. We propose the symbols Phr1 to describe the dominant resistance allele in 75A:258, and Phr2 for the incompletely dominant resistance allele in Merrit. Phr1 appears to be epistatic to Phr2, and expression of Phr1 may be altered by independently segregating modifier allele(s).