Two new species of Macrosiphum are described, both of which live in similar dry forest habitats of the inland northwestern U.S.A.: Macrosiphum dewsler n. sp. from Linum lewisii (Linaceae), and Macrosiphum garyreed n. sp. from Geum triflorum (Rosaceae). Information is presented about the habitats, host plant biology, distribution, and taxonomy of both species.