Several characteristics of the cultivated tomato (Lycopersicon esculentum Mill.) and several wild relatives were explored as factors in resistance to the pink form of the potato aphid. Foliage volatiles of resistant and susceptible plants were found to be qualitatively similar but quantitatively different. Olfactometric tests revealed that the aphids did not react in any detectable way to the aroma variation conditioned by these quantitative differences. Epidermal hairs (non-glandular) of the normal genotypes were not a factor influencing degree of attack by the aphids. In the field the aphids avoided both an excessively hairy compound mutant stock Ln-Wom and the wild tomato relative L. hirsutum. However, under infestation in the laboratory the insects managed to feed on these plants. Pubescence in the normal genotype is not a factor affecting resistance. However, increase of hair density and length tends to restrict aphid feeding activity under field conditions. The presence of anthocyanin in the foliage did not inhibit aphids from feeding. No anatomical obstacles to reaching the feeding site, the internal phloem, were found in resistant accessions of the green-fruited species L. hirsutum, however, thick cortex in the stems might prevent aphids from reaching vascular tissue. Comparative analysis of foliage of susceptible and resistant plants revealed higher sucrose, lower quinic acid, and higher alanine and tyrosine contents and a trend toward higher total free amino-acid concentration in the former. Furthermore, susceptible plants were unique as a source of o-phosphoethanol amine.