Abstract Resistance to the western biotypes of Aphis gossypii Glover in Cucumis melo L. breeding line LJ 90234 (inbred of P.I. 371395 from India) included tolerance expressed as freedom from curling of leaves following aphid infestation. The flat and curled phenotypes in progenies from a cross of this line with cv. PMR 45 were differentiated by a single gene, melon aphid tolerance (Ag, Aphis gossypii tolerance) with flat leaves dominant. Tolerance expressed as nearly normal ht of F 1 and some F 2 plants following mass infestation appeared to be less simply inherited. Its measurement was masked by inherent variation in growth rate and environmental factors including variation in insect attack. LJ 90230, selected from P.I. 161375 from Korea, was stunted by the aphid but its leaves remained free from curl. F 2 hybrids from the cross 90234 × 90230 were free from curl but they varied in stunting after aphid attack. Single-peaked distribution curves for ht suggested complex inheritance of tolerance. LJ 90254, selected from P.I. 255478 from Korea, possessed tolerance to aphids expressed as freedom from stunting and curling. F 2 plants from the cross 90234 × 90254 were free from curl but varied in ht after aphid attack. The single-peaked distribution curves suggested complex inheritance of tolerance.