One new species of the aphid genus Macrosiphum Passerini is described from the Pacific Northwest of North America. Macrosiphum opportunisticum n. sp. lives on a diverse range of shrubs mostly in mountainous habitats, without host alternation. The alate vivipara of Macrosiphum parvifolii Richards is briefly described, and the host range and biology of this species is discussed. The life cycle of Macrosiphum tolmiea (Essig) is reported to be holocyclic with notes on the morphology of the fundatrix. Macrosiphum kiowanepus (Hottes) is declared a junior synonym of Macrosiphum valerianae (Clarke) syn. nov. with notes on biology, host plants, and diagnostic features of the species.