The introduction into commerce of raspberry cultivars with major gene resistance to the large raspberry aphid, Amphorophora idaei, an important pest and virus vector on red raspberry in Europe, has been very effective both in decreasing pest numbers and greatly restricting infection with the viruses it transmits. However, biotypes of the aphid able to overcome these genes have developed in the field in recent years. Additionally, in field and laboratory tests, the response to aphid biotypes and recognised aphid strains of certain raspberry cultivars, such as Glen Prosen and Delight, differ markedly despite the fact that they are reputed to contain the same A. idaei-resistance gene, A1. In attempts to understand the reasons for this difference in response, analysis was made of the segregation of progeny seedlings from crosses between A. idaei-resistant and -susceptible cultivars to two recognised strains of the aphid. These studies showed that, as expected, cv. Autumn Bliss contained the A. idaei-resistance gene, A10, and cultivars Delight and Glen Prosen each contained the A. idaei-resistance gene, A1. When progeny seedlings were assayed in a heated glasshouse as young plants and in an unheated Tygan house as 1-m tall plants, the segregation ratios for resistance and susceptibility to A. idaei were largely unchanged. However, when the resistance of individual progeny plants was assessed, c. 37% of the putative gene A1-containing progeny and 9-23% of the putative gene A10-containing progeny, behaved differently in these two environments. Experiments involving an A. idaei-resistant and -susceptible parent cultivar showed that shading plants increased their susceptibility to A. idaei colonisation. Whilst this shading effect has implications for experimentally detecting A. idaei-resistant progeny in segregating raspberry seedlings, it does not explain the difference in field resistance to A. idaei of cultivars Delight and Glen Prosen. Such differences in the field seem best explained by the presence in these cultivars of 'minor' genes for A. idaei resistance and/or susceptibility that influences the effectiveness of gene A1.