Summary Cytogenetic study was carried out on willow aphids viz. Cavariella aegopodii and Tuberolachnus salignus in order to ascertain the karyotype and sex determination in these aphids from different localities of Himachal Pradesh, India. In Cavariella aegopodii, collected from Shimla, Mashobra and Shoghi localities, 2n is 10. There is 1 pair of long, 2 pairs of medium sized and 2 pairs of short chromosomes. However, karyotypic variation was observed in aphid populations collected from the Solan locality, where the variable diploid chromosome numbers such as 2nϭ8, 9 and 10 were also recorded. The male sexuals of Cavariella aegopodii are diploid having 2nϭ9 with 4 pairs of autosomes and a single X chromosome. The various stages of meiosis in the testes of the male mainly occur during the early developmental stages and can also be found in the first and second instar nymphs. The Cavariella aegopodii has XX-XO type of sex determination system.In Tuberolachnus salignus collected from the Shimla, Mashobra and Shoghi localities, 2n is 20. The idiogram of Tuberolachnus salignus reveals the gradual decrease in chromosome length. However, karyotypic variation was observed in aphids collected from the Solan locality where the diploid chromosome number (2n) ranged from 18 to 20.