The Rocky Mountain grasshopper,Melanoplus spretus(Walsh), was the most serious agricultural pest in the western United States and Canada before 1900. Immense swarms periodically migrated from the northern Rocky Mountain region and destroyed vast quantities of crops throughout the western and central portions of the United States and Canada during the mid-1800s. However, in the late 1800s, this species began a precipitous decline, and the last living specimen was collected just after 1900. There has been no satisfactory explanation for the sudden disappearance ofM. spretus. Macroscale changes in the western United States (i.e., climatic events, loss of Indians and bison, and the introduction of cattle) do not appear to have been of sufficient ecological intensity to explain the extinction. However, because the distribution of the insect was extremely compressed due to a natural population crash in the early 1880s, small-scale changes may account for the disappearance ofM. spretus. Within the region that this species occupied in the late 1800s, oviposition and early nymphal development occurred almost exclusively in riparian habitats. Intensive anthropogenic effects (i.e., tillage; irrigation; loss of the beaver; and introduction of cattle, plants, and birds) were associated with virtually the entire range ofM. spretusin the late 1800s. We suggest that localized agricultural destruction of the insect's habitat and the introduction of exotic species caused the extinction ofM. spretus.