Field collections of Amphorophora rubi (Kalt.), the rubus aphid, from 28 localities were classified into strains by their reaction to the raspberry varieties Mailing Landmark and Seedling 136/2, the former being resistant to strains 1 and 3 of the aphid, and the latter to strain 2. A new strain, numbered 4, was isolated, capable of breeding on both varieties, and it was found that the field distribution of this strain and of strain 2 was closely linked with the variety Landmark. Strains 2 and 4 appeared to have low fertility when inbred and, after a year of rearing in captivity, were all found to be homozygous for the genes conditioning their host plant reaction. Hybridization of strains 2 and 3 led to increased fertility and to the production, in the F2, of strain 4 types. An hypothesis postulating single-gene control of the ability to colonize Landmark and 136/2 is advanced and genotypes for the four strains are suggested. The implications of the discovery of strain 4 for plant breeding for resistance using the plant genes A1+A2 or A1+A3 are discussed, since strain 4 is capable of breeding on plants carrying these combinations. It is concluded that the low population levels of strain 4 are due to low fertility, and that the gene combination A1+A2 or Al+A3 should still be of use, particularly if field sources of strains 2 and 4 are reduced or eliminated by grubbing out any plantations of the variety Landmark.