Prosopis juliflora (Sw.) DC is a woody plant from the Americas that has dispersed worldwide via human intervention. Typically introduced with good intentions, Prosopis often proliferates and degrades native ecosystems. Prosopis first appeared in the Allideghi Wildlife Reserve (AWR) of northeastern Ethiopia in 1997. In 2005–2006 we determined: (1) patterns of Prosopis dispersal and establishment using global-positioning system mapping and seed-bank assessments; (2) impacts of Prosopis on cover composition and species richness of grassland vegetation using transects at replicated Prosopis stands that varied by tree size; and (3) attitudes of local people toward Prosopis using focus groups and interviews. Prosopis seeds first arrived in the AWR after pastoral livestock consumed seedpods along the Awash River, some 50 km away. Seeds have been deposited in corrals at recently established pastoral settlements within the AWR, and saplings now sprout along livestock trails. Prosopis has also colonized the AWR core grassland area, a vital habitat for wild grazers. Compared with sites lacking Prosopis, the largest class of Prosopis significantly reduced understory basal cover for perennial grasses from 68% to 2%, increased soil surface exposure from 30% to 80%, and lowered the number of grass species from seven to two. Attitudes of pastoralists toward Prosopis have become more negative over time. Local communities use Prosopis via limited charcoal production with some grinding of the seedpods for livestock feed. Infested sites are cleared by hand, but control has been ineffective. Because pastoral livestock are the main vectors for Prosopis seed dispersal and facilitate establishment, they will help transform the core of the AWR ecosystem from open grassland to denuded Prosopis woodland. While this bodes ill for grazing animals in general, it has particularly negative implications for the survival of an isolated population of an endangered, grass-dependent species—Grevy's zebra (Equus grevyi).