Tomatoes, an important tropical crop, suffer greatly from various plant viruses which are transmitted by the tobacco (sweetpotato) whitefly, Bemisia tabaci. Since these viruses cannot be controlled directly, the only path available to the grower is to prevent their transmission. The shortcomings of chemical control are well known and the use of alternative control methods is limited. Consequently, resistant cultivars would be the best solution for the grower. Breeding for resistance to the whiteflies as vectors of viruses has some clear advantages compared with breeding plants resistant to the virus. In search for sources of resistance among wild tomatoes, the two most resistant accessions of Lycopersicon pennella, L. hirsutum and L. hirsutum f. glabratum were selected. Although these accessions are susceptible to the virus, in the field they were only slightly, or not, infested. The resistance of L. pennella was found to be based entirely on the sticky material which is exuded by glandular trichomes located on the leaves and stems. The rate of resistance depends partially on environmental factors like photoperiod and light intensity. A reliable method for quantitative detection of resistance in L. pennella was developed.