Pinyon pine, Pinus edulis (Engelm.), in Northern Arizona is exposed to recurring high levels of herbivory by the moth Dioryctria albovitella (Hust.). During a 3-year period, infested trees experienced on average a 30 percent reduction in annual shoot production. This herbivory affects tree architecture, growth rate, reproductive output, and sexual expression. Less infested trees produce 47 percent more trunk wood, 43 percent more branch wood, and are monoecious. Architectural changes in infested trees can result infunctionally male plants due to a complete loss of normal female cone-bearing ability. When herbivores are experimentally removed, normal growth and reproduction patterns resume. These strong herbivore impacts should represent a potent selection pressure in the evolution of host traits.