SUMMARYThe inheritance of resistance to lettuce root aphid, Pemphigus bursarius, was studied in lettuce using the Wellesbourne cultivars Avondefiance and Avoncrisp as resistant parents and Borough Wonder and Webb's Wonderful as aphid‐susceptible parents. All four cultivars were crossed in all possible combinations including reciprocals and the response to root aphid of plants in the P1F1F2 and BC generations was assessed using apterae of P. bursarius from the lettuce cv. Iceberg. Resistance to attack was clearly inherited and the parents appeared to be homozygous for their resistance or susceptibility. In the F1 generation, however, in all crosses between resistant and susceptible parents, segregation into susceptible, resistant and some slightly less resistant plants occurred. This and the highly significant differences in segregation between pairs of reciprocal crosses in the F1 and other generations indicate that the inheritance of resistance to root aphid is controlled by extra‐nuclear factors. Modifying genes might also be involved but there appears to be no linkage of root aphid resistance with resistance to downy mildew, for which the Wellesbourne lettuces were bred.