The aphid, Aphis gossypii, is a primary pest of citrus, cotton, cucurbits and greenhouse-grown vegetables in Turkey and throughout Europe. There is some previous empirical data suggesting that host-adapted genotypes of this aphid exist which may in fact be host-races. To determine if host races of A. gossypii are indeed present in the eastern Mediterranean region of Turkey, reciprocal host transfer experiments and life table analyses were performed with multiple asexual lineages (= clones) of the aphid collected from different hosts. The collection hosts included citrus, cucumber, eggplant, okra, sweet pepper and cotton. Aphid developmental times on the host from which the aphid was originally collected (= collection or natal host) were shorter (5.2-6.0 days) and had a higher intrinsic rate of population growth (r(m) = 0.25-0.44) than the 6.6-7.3 days required when the aphid was reared on a non-original collection host (= non-collection host or non-natal host) and had r(m) = 0.03-0.30. Total immature mortality of the cotton clone, especially in the first nymphal stage, was high (51-100%) with low r(m) (0-0.03) on cucumber, citrus and sweet pepper. Aphid populations transferred from citrus, eggplant and okra to cotton (r(m) = 0.29-0.30) did not differ significantly in their performance from that of the cotton population on cotton (r(m) = 0.34), whereas that from sweet pepper and cucumber populations (r(m) = 0.22-0.24) were significantly lower. These data have allowed us to separate A. gossypii into two distinct biological groups: (a) a 'generalist' population obtained from cucumber, sweet pepper, citrus, eggplant and okra which exhibited statistically better development on cotton; versus (b) a population from cotton which, by comparison on reciprocal hosts, developed poorly on non-natal hosts except on eggplant. Development of the cotton clone on cucumber and okra was not improved after four successive generations on the non-natal host. The good development of A. gossypii from eggplant and cotton on these reciprocal hosts suggests that these particular clones were similar, if not identical, host races.