The tobacco whitefly, Bemisia tabaci Gennadius (Homoptera: Aleyrodidae) which occurs in various parts of the world, has developed a high degree of resistance against several chemical classes of insecticide, including organophosphates, carbamates, pyrethroids, insect growth regulators and chlorinated hydrocarbons. The present studies were done in order to monitor tho susceptibility of whitefly populations in southern Spain to insecticides commonly used there. Systemic bioassays using Spanish field populations of B tabaci collected in 1994, 1996 and 1998 indicated an increase, albeit a slow one, in resistance to imidacloprid over this period. Comparative studies of other neonicotinoids using the same bioassay revealed a high degree of cross-resistance to acetamiprid and thiamethoxam, Leaf-dip bioassays with adult females from these populations revealed a high level of resistance to cyfluthrin, endosulfan, monocrotophos, methamidophos, and pymetrozine, each at 200 mg litre (1). Buprofezin and pyriproxyfen were tested against second-instar nymphs and eggs, respectively, Buprofezin also showed a lower efficacy against ESP-98, a strain of B tabaci received from Almeria in 1998, but pyriproxyfen resistance was not obvious when tested against eggs of strain ESP-98, Field trials in 1998 revealed good efficacy of imidacloprid in one farm In the Almeria region and two greenhouses in Murcia and Sevilla, but a loss of activity by imidacloprid In another farm in the Almeria region. Cross-resistance between imidacloprid and thiamethoxam was also confirmed under held conditions. (C) 2000 Society of Chemical Industry.