Holocyclic populations of aphids of the Chaetosiphon (Pentatrichopus) fragaefolii (Cockerell) group from Fragaria in British Columbia were found to have a homozygous 2n = 12 female karyotype, whereas samples from predominantly anholocyclic populations in Oregon, California, England and New Zealand have 13, 14, 15 or 17 chromosomes showing various degrees of structural heterozygosity. The 17-chromosome form is a morphologically recognizable taxon, C. jacobi Hille Ris Lambers. The other karyotypes apparently represent a series of parthenogenetic lineages derived from the 2n = 12 holocyclic form and carrying one, two or three autosome dissociations. The North American populations showed no correlation between karyotype and the number of submarginal setae, a character previously used in attempts to differentiate taxa within this species complex in western North America. Submarginal setal number varied greatly within and between clones and tended to increase in successive generations from the fundatrix, confirming that this character varies both genetically and seasonally and has little taxonomic value. Populations of the fragaefolii group from Rosa spp. in North America have a shorter rostrum than those from Fragaria. The name C. thomasi Hille Ris Lambers is assigned to this rose-feeding species, which is holocyclic on R. rugosa in British Columbia and colonizes Potentilla spp. (but apparently not P. anserina) as alternative hosts. Old World populations on Fragaria, presumably originating from North America, have longer siphunculi than North American populations of fragaefolii and few or no submarginal setae; these differences are probably due to founder effects. Both Old and New World populations of fragaefolii use P. anserina as an alternative host-plant.