Fossil aphids found in 13 pieces of Cretaceous Canadian amber from Alberta, age 73 million years, are described, and their morphologies, systematic positions, and biologies discussed: Cretamyzus pikei Heie, gen.nov. and sp.nov., Mesozoicaphis electri Heie, gen.nov. and sp.nov., Mesozoicaphis tuberculata Heie, sp.nov., Mesozoicaphis canadensis Heie, sp.nov., Mesozoicaphisparva Heie, sp.nov., Calgariaphis unguifera Heie, gen.nov. and sp.nov., Albertaphis longirostris Heie, gen.nov. and sp.nov., and Campaniaphis albertae Heie, gen.nov. and sp.nov. Cretamyzus has been placed in a new family, Cretamyzidae, within the superfamily Aphidoidea, and the last four genera are placed in a new family, Mesozoicaphididae, within the superfamily Phylloxeroidea. It is contended that the origin and diversification of angiosperms occurred in the Cretaceous, resulting in extinction of several old specialized aphid groups feeding on gymnosperms while adaptive radiation of some less specialized and species-rich aphid groups occurred. The main part of the previously described Cretaceous aphids belongs to families that became extinct at the end of that period, and the fossils known from the beginning of the Tertiary already show a remarkably large resemblance to recent aphid fauna.