Population of aphids on a melon cultivar, PMAR No. 5, was much smaller as compared with that on other cultivars. Incidences of leaf curling and stunting caused by aphids were not prevalent in PMAR No. 5 in both mass and isolated infestations. These results indicate that PMAR No. 5 is resistant to aphid. On resistant plants, aphids could proliferate slowly, and migrate rarely from others. It is very likely that antibiosis and nonpreference in melon plants operate as resistance. The resistance is genetically a dominant trait governed possibly by a single major gene. Aphid resistant plants could be selected on the basis of a leaf curling index with a supplementary information on aphid population size resulting in ten days after aphid placement in a mass infestation test. Aphid resistance manifests itself on the above-the-ground part of the plants, having no connection with root-related substance. No relationship was seen between the trichomes or substances on leaves and the aphid resistance. In the field tests, plants possessing aphid resistance were almost free from infection of virus mediated by aphids, and use of insecticides for aphids was generally not required. Several inbred lines were derived from the hybrids between PMAR No. 5 and Japanese cultivars of an EARL'S FAVORITE type. Some of the F1 hybrids, using those inbred lines as parents art expected to be released as commercial varieties in Japan.